I like the idea of a "Guest remote" but there are some problems I can see with this.
Firstly, some sort of access control would have to be implemented to only allow guests to add to the current playlist, rather than making any changes to it - and I can see that being quite frustrating if they accidentally add a full album rather than a track, or select the wrong track, for example.
I suppose you could, in theory, track what changes a specific guest/device has made, and allow them full control over their own additions, but it seems like the sort of thing which leads to headaches.
Perhaps giving them full control, except being able to clear the entire playlist in one go might be the solution? These sort of control restrictions just seem problematic.
Another issue is actually giving them access. The first thing is that they have to be on your WiFi network - OK, if they are going to be staying a while, or they are frequent visitors, I suppose they would already be on the network.
The bigger issue I see, is that apps like JRemote cost money. If they aren't running Media Center at home, and don't already own it, it seems unreasonable to expect them to buy an app for this. (maybe if it were 99 cents that would be acceptable)
An alternative to an app would be a web app that they access via the browser - and I think this would be a better solution.
WebGizmo works, but I have problems with giving my guests access to it. You have to log in, so once you've done that, they now have worldwide access & control of your home setup. (assuming you're configured to allow external access)
Perhaps the best solution is "guest mode" access via WebGizmo, running off a separate account.
If there was a separate Guest login, which only allowed internal access to the library, with completely separate library views (I don't necessarily want guests to have access to the entire library) it could work.
Personally, I wouldn't have any problem with guests being able to clear or replace the playlists, but I might want to restrict what zones they can play to, and what content they can view.
E.g. audio only, and only control over the living room and kitchen zones, rather than whole house control.
Right now, the easiest solution is probably to just hand them your device, or keep an older tablet around as a dedicated remote.