I think that as long as the drain/fill times remained a constant, it would probably work.....but, I wouldn't do it if it were me. I would want the pump on/off circuit actuated by something more directly influenced by the water level itself, rather than elapsed time.
I still like the float idea, and I still believe it could be accomplished safely and reliably without taking out a second mortgage. It just seems to me to be the best fit for the situation at hand.