I think this would be a good idea. I've made several houses/buildings with floor drains in the basement/garage/restroom, and right now in vanilla all I can use is Trapdoors, or a note block to get the desired effect. Plus the On/Off could be used in a swimming pool, as long as you have a way to quickly fill it again if it's a big/deep one.
Am I the only one to notice the recipe is now taken up by the iron pressure plates? But besides that, seems like it could work. Depending on how sorry a state the current water engine of Minecraft is in.
So... is this idea going anywhere anymore? Or is there something with grates already out there now? Either way, this is an amazing thought, and something I would use all the time. I've often tried to use workarounds for simple grates but it's rather annoying.