Users and groups is an entirely different realm but I've wanted that as well. An integrated way to show certain content would be good. The problem is that it could easily get way too complicated. I am thinking something as simple as a general default user plus the ability to switch users to add certain excluded content.
At least in theory, I don't believe it would be that hard to include a user or group option to MC. My first thought is to only go for groups or users. Both might be to overdo it. Normally you'll have 1-4 scenarios of what media you want to display or be able to write to, so there's not a need for an elaborate
Music: Limited read access to Music only (kitchen or bathroom HTPC for instance). No write access and only Music files shown
Family/Friends: Full access to media, but with write disabled (shared library with family or room mates). Prevent Number plays update etc.
Power User: Main HTPC with a primary user. All access and write access
The problem with updated Playing now, Last played etc, would be best to solve by implementing a small Library part with independent user data, so each user have it's own data for specific tags.
The easiest way of using this access control might be to set the read and write access on media files, and to hide the playlists, view schemes if there is no files present. If this is controlled on each file you'll have to have a way of setting the defaults for each category (music, images, video and data), so you don't have to do a manual job for every import.
When you do want to micro manage the access rights, mark files, de-select the groups for the individual files you don't want other users to see.
I don't see a big problem with this, but there might be some lurking around.