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.

On the other hand, people have been exploiting whatever game mechanics are available since day one.