MWSE can do that can't it? xGetRef I think is the function. You could even do a string match to check names for partial match and assign a certain amount of xp to that type of baddie or that special character.
And add to that, divide it by your level so that the higher the level you are, the lower amount of exp you get based on the baddie! Would keep people from abusing it and prevent over leveling....
Though, to be honest, Morrowind's leveling system is still kind of xp based, only instead of the xp working towards levels directly, it works towards skills, and those skills then effect your level, or at least thats how I always looked at it.
Personally, I like how using long blade ensures that your long blade skill increases as you level, as opposed to a generic stat increase every level regardless of what your character is actually skilled at, but to each there own, and I still think this idea would be good for anyone that wants a system like this and is willing to stick with just vanilla creatures/NPC's as I could see Mod added creatures/npc's breaking it.