impossible because it's how the game is made. unless Badspot changes something (99,9% chance of not happening).
What about modifying serverCmdPlantBrick to check if the ghost brick is a water brick and if so planting it anyway if there aren't any water bricks in the way, creating an artificial third layer?
It could be possible, but it'd be difficult.
It would also mess up faces on bricks frequently.
You would also have to fix loading and possibly saving. Not sure.
I might make something like this one dayOr someone could make a water brick that plants like a normal brick.