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"?