keyplyr - 7:09 am on Feb 19, 2012 (gmt 0)
Keep your sitemap.xml and just add the forum as a subdomain (with respective A record change) Example: forum.domain.com.
Then add a separate sitemap.xml for that subdomain which could be tagged for hourly changefreq even though you would not be updating the time stamp.
Not quite perfect, but close. Google just uses sitemap.xml as a suggestion anyway.