I am awed at this suggestion. I see no reason why this could not work. Firstly, there are already 2 types of water. one more can't hurt and the current system could even be adapted from frozen/active to falling/stationary without adding a new block type. Secondly, I think this might actually require less processing power per tic than the current system. I can't totally prove that, but since it creates the blocks over a longer period of time and the most active blocks only need to check one face.
Thinking about this I also thought of a new and better anti-griefing tool: The anti-flood block. Just like sponge it has a field around it. If this field comes in contact with falling water it replaces the falling water with an active anti-flood block. First, the active anti-flood block replaces all active water on the edges of a 7x7x7 cube around it with active anti-flood blocks. Next, the active anti-flood block replaces all the ground water, falling water, and other active anti-flood blocks in a 5x5x5 cube around it with air. If there is no water or anti-flood blocks in the 5x5x5 cube at all the active anti-flood block turns into sponge. If I am right that means you can place anti-flood blocks beneath the floor of your underground creations and if they are flooded all the water up to and just past the source of the leak will be removed and held back with a regular lattice of sponge blocks.
I'm bored and have nothing to code tonight, so I will hack together a 2d representation of all the ideas posted in this topic so far and try to answer some of my own questions. I'll do my best to post the results and hopefully the program when I'm done.