A default playlist would certainly fill a potential gap in the solution. Or just having the ability to turn off all Theatre View menu items with a keyboard shortcut or better a remote command available through MCWS. That way a background Theme slideshow would play when nothing else was playing. Maybe just editing a skin to make all menu text 0% opaque would be another solution.
I would also use Zones, because you would want the Playing Now list, created by the video Playlist, to remain intact while you switched to an ad-hoc video. But I think it would work best if the Playlist Zone could be paused rather than stopped, and then an ad-hoc movie started in a second Zone, and then the user switches to the second Zone. But if the first Zone is paused, and if it used an Exclusive mode for audio, audio won't work in the second Zone.
I would expect a Theatre would want to use WASAPI or ASIO Exclusive Mode for the best audio, so that would leave having to stop playback in Zone One, and then switch to Zone Two and start playback (or start playback in Zone Two and then switch to it, my preference.)
But if the Theatre View menu items could be hidden, then a user could Stop playback in Zone One and drop back to the Theme background slideshow, switch to Zone Two, and then start playback in Zone Two, which would probably look pretty good.
That could probably all be done with an existing remote solution, or using any external control application which can send MCWS commands. But it may need some additional automation so that a user could just play a playlist, then at any time select an ad-hoc video and play that, then when that finishes or is stopped, switch back to the playlist and start playback where it left off.
Like I said, interesting.