I was looking around on Bioware to see if I could find an answer to a question when I came across this: A script enhancement to allow more than one henchman. It's not totally completed yet (as far as I could tell anyways), but thought you guys might want to know it is out there. Also, in the thread I read about it initially, there may be an epic level enhancement already out. I'm gonna do some more research and if I find anything will post it here. So. Those characters above 20 may be able to continue raising levels.
Original thread where I read information (Note that a Bioware mod is a part of the thread):
http://nwn.bioware.com/forums/viewto...95311&forum=42
Download page for enhancement script (Note that a better link may be in the thread, just that this looked easier in one spot):
http://www.demons.org.il/evilspice/NW.htm