I thought about this a couple years ago. I never did build it, but was set on the idea of mounting a water barrell/spray tank above my box blade. A PVC tube to a "T", and more tubing to spread out from side to side. Small holes drilled in the tube that spans side to side. A valve somewhere. Let it drip onto a couple layers of beach towel when open. So, using it, I would first leave the water off, using the boxblade to cut off the high spots and drag the ice parts to the side of the rink. Might be real nice to have a rotary brush on the front of the tractor at this point too, but hopefully not required. Then go over it all again with towel dragging and water dripping. This should apply a thin coat of water. It will self-level, and should freeze real fast if its just that thin layer.
PLEASE keep us posted if you build anything.