Personaly I havent thought about it this way.
My solution was to spread zones into 5 level tiers, and assign idividual XP modifiers to them.. Thought in my case they would be NEGATIVE
Since I belive players level up way to quickly =)
But I did had somewhat related idea to introduce XP caps on zones. For example: Lower Guk will have a max XP cap of 50 - which means once you hit 50 - you WILL NOT get anymore XP in this zone regadless what you kill. This will allow to on one hand boost (or lower) XP bonus for specific zone, but on other hand once players go beyond intended level - they will not be able to keep staying here and rip good XP chain killing mobs weaker than them.
Thought my original intention was to use to kick players out of the zone past certain level, so they ALWAYS forced to fight mobs close to their own level to get any Xp at all.
You can read my entire proposal here. Still trying to get devs to implement it
(hey it was even approved by KLS!!!! )
http://www.eqemulator.net/forums/showthread.php?t=24648