Not that I want to encourage background tasks or that you would want to add them but what about just what I said, a very slow, idle task that built thumbnails when idle, one at a time when not busy?
Or, I have many user accounts / libraries / systems that use the exact same server library--basically three folders: /music, /video, and /pictures
Would it ever be possible to store the thumbnail database on the server in a central location I choose? Can you get satisfactory thumbnail performance over a gigabit or 100Mb network connection? Then, the server could handle all thumbnail building and all clients could easily share the one database. It's a lot of work building thumbnails for a huge library so many times.