You might be able to edit your classes and templates related to the 'forum', replace your forum attribute 'Message [Text block]' with 'Message [XML block]' and enable the online editor for your user siteaccess.
I'm not certain just the best way to limit display of online editor controls which you do not want to offer to forum users...
Sorry, I couldn't find where to modify forum attribute:
You might be able to edit your <b>classes</b> and <b>templates</b> related to the 'forum', replace your forum attribute 'Message [Text block]' with 'Message [XML block]' and enable the online editor for your user siteaccess.