There are two antispam measures that affect Tor users:
1 The web server checks your ip against the Http:BL black list to deny or allow the site to be browsed, but the most usual TorBrowser useragent is explicitely whitelisted, so it should work.
2 Then the registration form also checks against a different blacklist (stopforumspam), so you may get stopped there. If so, requesting Tor to use a new identity (in the vidalia panel) usually does the trick. You may need to do this more than once if you are not lucky.
Once registered, you are just checked against 1, and as explained if you use TorBrowser it should work (don't use anything else to browse Tor!). If not please contact through the development list or the contact form.
