said by BeesTea:Thanks to all who commented regarding the iTunes content. I suppose having to alt-tab between plex or xbmc and a full-screen iTunes while not ideal isn't too terrible.
Thanks!
It would seem to me that I could write a program that would get the contents of the iTunes library, either from iTunes or the .xml library file, then, when asked to play an iTunes file, launch iTunes playback of it.
I just chained the "Get Specified iTunes items" action with the "Start iTunes Playing" action and got iTunes to play both a DRM protected song and a movie (I don't own any DRM protected movies). Both played just fine and the movie started playing full screen.
I don't know why plex doesn't just write a little applescript add-on that enumerates the iTunes library and then uses the script to playback things in iTunes.