To come back to this.
Is your installation like
www.xenforo.com , where the forum is in a subdirectory?
And now you want to have the portal content on
www.example.com and the forum and everything else stays as it is in
www.example.com/forum ?
I'm curios how many other users would be interested in such a feature?
If there are enough, it would be worth to rewrite Simple Portal, if not, it would be released as 2 additional addons