Yeah, it's possible. My zombie XP farm causes the zombies to fall 22 blocks so they're only 1-3 punches away from death (a few die in the fall for some reason). I let them build up for half an hour or so, and when I come back there's hundreds of zombies waiting to be punched in the knees. I could also use splash potions on them to kill the whole group in one go, but I haven't tested that yet.
Ah, but you still have to attack them yourself to get the XP.
I suppose the traps do make it a bit too easy.
On the other hand, people have been exploiting whatever game mechanics are available since day one.