I have installed XF in the root directory at mysite.com, however I would like the forum homepage to be set/accessible via mysite.com/forum and I want the root of the domain, mysite.com to display a XF page or thread.
Why not? Because it is a pain to do or that he feels it is not useful? (If the latter I need to try and show him it is - hopefully my post above does..)
If you get it out of the query string via an add-on, you could then use a route filter. You'd need custom processing on the forum route prefix to handle everything though.