I own the trackbikes.ca domain for some time now, which I bought with exactly the same idea in mind. But I've been bitten by the lazy bug lately and I don't see when I'll recover from it to do some development and do it properly...
I'd be happy to work on something with you in collaboration... I have trackfanatics.ca, you have trackbikes.ca, I work with Gareth over at trackdayhub.com.... you think we could come up with something. Talked to John Brundl a while back and he says he just doesn't have time or impetus anymore.
As a specific sub-forum on here, or the "bikes for sale" section, i think it may be worth exploring. However, i'm sure every sub forum costs money so it's ultimately down to the site owners.