Why can't you have a persistent connection? It's possible to have multiple persistent connections to databases, just ensure that the link identifier is correct for the connection trying to be made.
The overheads won't be bad once it gets implemented, and you should be able to set up a cron job to clear overheads at 12.00 every night or whatever, whenever traffic is lowest to the site.
The data doesn't need to be split manually, run a script that reads from the database all data for a particular forum, moves it to the new database, and deletes from the original database.
I think that would work, if you can see any other reasons why it wouldn't let me know and i'll try and come up with a solution.
(BTW: im not trying to be a smartarse or a know-it-all, because I could be wrong, i'm just trying to help.)