[thinks aloud]
You could make the 'owner' of the branded forum a mod. This will give them access to the forum settings.
You (at this point) would have to create a style they could use and assign it to that forum.
Catches that come to mind are:
When they register, they will be sent to the forum summary page. This may not be too hard to fix. possibly by setting and additional cookie paramater based on where the register option is selected (IE, which forum they are in) and add some code to pass that value into the the title image and so on.. (url variable forced to a value in cookie) Also additional options in ultimatebb.cgi to get the 'from' address and direct them to the right forum.
Templates would make this 'fairly' easy but a major overhaul anyway.
Would be a good bit of work but it is definatly attainable. I bet 80% of the coding would be done to the templates.