Your best bet is probably to switch to using the Library Server system now, instead of directly connecting the HTPC to the main Library. The Library Server system still isn't perfect, but it is VASTLY improved over older versions that you're probably used to from MC14.
You didn't mention if the Desktop PC is always-on or not, but if so, the way I'd probably handle this is:
0. Set up MC on the Desktop PC with a Library on the local disk.
1. Run MC in Library Server mode on the Desktop PC. Set Options -> Startup -> Windows Startup -> Run on Windows startup: Media Server. Then enable Options -> Media Network -> Use Media Network to share this library, and enable the Authentication option. This should be enough, but you can also modify the TCP Port used if you need to under Advanced in the same section of Options.
2. Connect to this newly shared library on the HTPC using the built-in Library Client system. This comes down to simply Adding a new Library and either entering the IP address:Port information for the Desktop PC, using the Library Server Access Key created in Step 2 above, or if you have working DNS/Static Hosts on your network using that instead. I think MC will even actually poll your network subnet looking for servers too. The most reliable way to do this is to make the Desktop PC have a static IP inside your network and point directly to it (this will also allow you to use port forwarding on your Router to get access to the server from outside your LAN if you want to down the road).
3. When connected to the Library Server via a Client (on the HTPC), you will have full access to the library, and CAN apply tags just like you would on the Desktop PC. Both can be up and running and connected to the Library simultaneously without issue. This is what has changed from old versions... The Clients attached to the Library Server do now have the ability to write tags. This will primarily just be things like Play Count and Last Played for your purposes, but it is nice if you want to tag something quickly from the HTPC while you are sitting there (or maybe rate some files with some stars and things like that).
4. On the HTPC, go to Options -> Media Network -> Client Options and enable the Play local file if one that matches option and also enable the Automatically sync changes option (these might default-on, I'm not sure). Please note: You MUST enable authentication for the Automatically Sync option to work correctly.
Lastly, and this may not be necessary for your needs, I want to review the things that DON'T work from a Library Server client. Basically we're talking about anything that needs to directly access the files on the filesystem outside of MC won't work on the client. This would include:
a. Right-Click -> Locate -> On Disk (external)
b. Right-Click -> Send To -> External/Web/etc
c. Drag-dropping files from MC's interface onto the Desktop or onto a Windows Explorer window to create a copy of the file.
d. The Album Art thumbnail in G-Force on the Client Machine. The thumbnail is pulled directly from the file on disk, so G-Force pulls a generic thumbnail (hopefully Andy updates G-Force to use the "proper" MC command to pull the cover art from the MC library in the future).
Those are the main things, though there might be a handful of other minor-but-similar things you'd encounter. The reason that these things don't work on the client is that if you look at the [Filename] "tag" in MC on the client machine, instead of listing the actual file path, it is a special internal URL-like locator for the file. MC does directly access the files for playback, if it finds a matching one (hence making sure that option is ticked in step 4 above) but this is basically just for playback, and hasn't been extended to the other filesystem manipulation tools yet. Hopefully in the future. This system allows you to use MC out from the Internet even when you don't have access to the big file-storage drive, because if it doesn't find a local matching file, it streams the file directly from the Library Server machine (transcoding as necessary).
When you're on your LAN, and you have your UnRAID drive mounted on the Client, it just plays the files directly without transcoding. It just isn't smart enough to do the same thing for items a-d above. This really isn't a problem if you don't do much/any file maintenance from the HTPC at all, and you don't need functions like I described in A-C. If you don't need those on the HTPC, then you can stop here and you'll be quite pleased with the system.
However, if you DO want access to those few functions occasionally, there is a way to work around it, if you have your Library on a "shared drive" on the server machine. Ideally, you'd have this directly on your "media drive" since this is already mounted and accessible on both of your machines, I assume with matching disk letters. That's what I do at home. My "media drive" is a big RAID shared on the network and mounted everywhere as drive M, and the library lives at M:\library_data\<library_name>\ and I have the entire M drive shared out over the network and mounted on all of my computers at home on the LAN.
Unfortunately, the one big problem with those NAS devices and UnRAID boxes is that they don't provide full, unfettered access to the filesystem actually on disk to any Windows machine on the network. Most commonly, the files themselves are stored on a Linux-type filesystem, and then something like SAMBA provides network access to these files for Windows PCs on the network. Samba is NOT the most reliable thing on the planet, which is why Apple is supposedly abandoning the open-source SAMBA stack on the forthcoming Lion version of OSX. It mostly works, but it has been reverse engineered and can be a little shady around the edges. The most common problems I see at the office with our SMB shares is with write permissions and locked files, and I suspect that's what you're seeing here. Things don't always "unlock" when they are supposed to, and sometimes this can cause data loss if you aren't careful.
So... Keeping the Library on the NAS/UnRAID is probably a bad idea. So, what could you do if you want access to items A-C above every once in a great while (probably not D, but that may not even apply to you), but you don't want to access the library directly from the HTPC anymore most of the time? Well, you have a simple option:
On the Desktop PC, share the folder where you saved the Library via the normal Windows network sharing system. Since this is a "real" Windows Share, and not a "hacked" SAMBA share, it works correctly (besides, you'll be using it in Read-Only mode 99.99% of the time anyway). Then, in addition to the Library Server method you used to add your library on the HTPC, add THIS Library DIRECTLY on the HTPC as well (call one "On Server" and the other "Direct" or something clear like that). For the Direct mode setup, you can simply point directly to the UNC path of the share, such as \\DesktopPC\Library_Data\ or whatever. Whenever you need to do something like A-C above, you simply go to Playing Now in the tree and switch to the "Direct Mode" library. It will load Read-Only, because it is still open on the Server, but that's no big deal if you just need to drag a bunch of files over to a folder on your desktop for some reason, and then you'll switch right back to "normal" On Server mode.
This is what I have set up on my systems at home, and it works great. The VAST MAJORITY of the time, I can just use the HTPC (or my Laptop) in Client mode. Every so often though, I want to grab some files directly (usually, actually, when I'm going through Photos with my wife and picking out ones to print at Mpix.com - this is nice to do on the big HTPC screen, and then I just drag the ones we select over to a "print" folder on the HTPC's desktop). When I need to do this, I just switch the HTPC over to "Direct Mode", acknowledge the Read-Only warning, and go about my business. The only hard part is remembering to switch back when you're done and to not try to do any tagging while you're in the Read-Only "direct mode". Hopefully they improve the Library Server and reduce/eliminate/work-around items A-D in the future, but that's where we are now.
Once you do this, you'll see some great advantages. One of the best is access to the WebPlay and WebRemote functions. This is especially cool if you have a smartphone or tablet of some kind, but it works even when you're over at a friend's house or something on their PC. If you make sure to forward that TCP port through your firewall, you can get to all of your music and photos ANYWHERE that you have internet access with a nice, easy to use Web interface. If you are outside your LAN (with a Laptop) and that Laptop happens to have MC installed? You can even use MC just as though you are at home, just a little slower (and it has to transcode the files on the fly as it plays them). Video streaming out over the Internet works too, but it is certainly a bit more "beta-quality" at this time (some file types work great, some don't).