Not to throw cold water on this idea… but it was an idea I had 4 years ago, and which was eventually implemented by a guy named Ryan, who used to post as the Guru (or Gambling Guru). It was called the Collective Gambling Forum, and used phpbb with mods to make the forum appear unique for each site, inclusive of logo, but the posts were all shared.
This forum died a slow death because it was extremely complicated to manage and they didn’t realize enough traffic and weren’t unique enough.
If you do go ahead with this idea, I hope you are all willing to contribute some uniqueness to the forums, otherwise you will head down the exact same path.
For the record, Got2Bet did not join in the forum in the end because it ultimately did not meet the unique needs of the site.