Actually, the XP bars are under a control called PLAYER_PROGRESS_BAR. It has different types for XP, CP, skill, and VP (still holdover for some reason):
http://esodata.uesp.net/current/src/...ssbar.lua.html
There are a few places you might be able to hook. I looked at them a bit for ChampionBar, but wasn't able to use any hooks due to code initialization ordering.