You're gonna have to start digging for that code.
Try to figure out what files are being used when a user posts a topic or replies. I would say the
modules/Forums/posting.php file would be a good place to start looking.
Then I am not sure, but you may have to alter your phpBB template files accordingly as well.

You desire a feature that I doubt anyone has looked at doing yet. So how about you being the poineer in your quest?
:wink:

Search for 'redirect' in that file.