Since Fossil wiki page support comments, a forum interface could be written in Javascript. Each forum would be a wiki page. Adding a new topic would add a comment to the forum page, referencing the new topic, then add a topic page. Replies would then be added as comments to the topic pages.

Of course, given that Fossil is a developers' tool, I would expect most discussion to be attached to tickets, so really only need a simplified way to append new comments to tickets.