Ok, found the problem:
Apparently the IC2 pump + Compressor combination only works when the pump is over what Tekkit considers a "Water Source" (Light-Blue water block). As soon as it is powered it pumps to fill its internal tank, and all is well. The compressor adjacent to it steals a large amount of that water and creates a snowball. Then, the pump pumps to refill its internal tank just as it should, BUT, here is where the problem comes in: Upon doing this second pumping it turnes the Water Source block into a "Still Water" (darker blue) block, and being over a Still Water block doesn't trigger the Compressor+IC2-Pump combination to make any more snowballs (I gave it 10 minutes to see, and it did nothing, and both had full power). So logically I thought that I just needed a Water Source block that would replinish itself as a Water Source block, so I searched over a few dozen in a large river on my map and found none that would do so. Next, I thought that this was in place to prevent the combo from being used too easily, so I went to the ocean thinking there would be perpetual Water Source blocks there, but alas, the water there acts identical to that in the large river.
So now I'm thinking that there must be some option selected in a config file when generating the map that allows Water Sources to stay Water Sources when interacting with an IC2 pump, or something else simple that I'm missing. Any ideas?