Idea is nice, but we could use categories/userpages for that. Dont forget, we are just another wiki.
and some people do use thier userpage for that, but the vast majority don't.
we need to get the home wiki information at signup time because with drive by users we may not get another chance.
i think we may even need to go as far as making it mandatory to specify username, password and wiki for an account on another wikimedia wiki at signup time. If we don't make it mandatory then the very people we need the info for the most are the type that are least likely to give it to us.