Creative solution needed: Splitting a site in two, how to redirect half?

[sub_ubi]

I need some help brainstorming.

One of our subforums has outgrown the rest of the site, so we're splitting it off onto its own domain to become an entirely separate site.

Here's the tricky part: we want to redirect every thread from that subforum to the new domain, but leave all other threads/forums intact. There's nothing in the thread URL which states the forum it's from, so a simple redirect won't work.

There are tens of thousands of threads in that subforum. All ideas welcome