Ha ha, I feel very proud of myself right now.

I just figured out that custom animations for texture packs run at 20 frames per second.

So, yes, I just figured out the whole custom_terrain/item_### thingy. Thanks to this awesome person:

http://www.youtube.com/watch?v=a4yrgViA0ZI

I had spent days on trying to find something like this! Thanks, Mikemc!