I got an idea for improving efficiency of an XP farm, but I don't know how viable it would be.
Considering Snow farming is easy, gathering a bunch of snowballs is quite trivial. They could be launched manually or through a dispenser with a pressure plate, or any other means. Augmenting the sky farm that way would significantly raise the number of zombies spawned, and as result, the farm efficiency. The question though is – does the zombie have to be damaged or merely hit to perform the "summon"?
Best Answer
A hit with a snowball can summon a reinforcement zombie.
Just tested with the following setup: 1.6.2, creative, hard, clean superflat world, time 23000, daylight cycle off. Time 23000 prevents both natural spawning and sun damage.
It can take quite some time to get a spawn, though. Wiki mentions 80 x 80 x 80 spawning area. If this is a randomly picked point, the probability is 1/80 per hit on unobstructed superflat, and it definitely is somewhere in that order of magnitude by my testing. Not worth it for farming improvement, I'd say.