I like that idea. Then of course you'd need a default setting for what you want to do with anime that you haven't set.
Probably nothing to really worry about in terms of server space - if each of 100,000 users sets 100 title preferences, we're talking an additional 50MB (assuming you have to store a 4-byte aid and a 1-byte flag) - and it would be cool. (22.5MB if you can get the flag size down to 2 bits, and only use a 16-bit int for aid). Could be significant, but I doubt it.
Currently the mylist sort order would have no problem handling mixed title languages - japanese characters first in japanese order, then roman characters in english order.
Maybe there should also be an option to hide other titles from the anime page once you've chosen one. And of course, it would show this title (or your default if nothing's set) no matter who's mylist you are looking at.
