Why?
Doesn't this idea (hdd intensive process running in the background) collide with your reservations from Discussion of Windows Indexing / Windows Search 4 where you would not let the Windows indexer do its duty as this slows down your system.
The feature to first load the list, then watch it and constantly update it is something completely different.
So all in all I still would say: it is already possible to get MP3tag to load files at the time of beginning of a session.