You can write simple workflow event (triggered after publish) then setup new virtual forwarding (look on code in the kernel/content/urltranslator.php). The process will be automaticaly. But when portal will have 20 000 registered users the list can be quite long ;)
mod_rewrite should do the trick. With one additional rule you can redirect users to the user group:
This will work but can cause problem when you have e.g folder with the same name pattern (aaa_bbb), so unique pattern for user object name should solve it (and pattern for re_write which match it). Try extend this example.
