well, all you need would be the DB_site object.
and then editing newreply.php
with db_site you run a query, and find out, how many lounge posts they have (as you have to join post and thread table, it is not the fastes query, but in newreply, you can do so) then devide it by $bbuserinfo['posts'] and then if the ratio is ok, allow them to post, if not then use the error function