Well whenever you died after going below this amount of xp while being 50 you would have the option of going back a level. This would keep someone from permanently dieing that didn't want too. The penalty would be a chance of losing hp when they got the level back.
The most hp you could get back would be what you got the last time you leveled, and you could get the minimum. If you dont think that is enuf pen you could make it a straight 10 hp loss or something.
-krynn the absent minded ambassador of Kwan