Is there a wiring design that would protect a person in bath from electrocution when a powered appliance is dropped?
Yes, but ...

1. But: One can always dream up more and more bizarre ways of killing yourself and then ask how to make it safe. "I am handing underneath a speeding stage coach as it ...". This falls in that class. Most regulatory authorities prohibit the use of portable high power electrical equipment in bathrooms and also prohibit the installation of other than special low power and isolated electrical outlets, in order to head off things like hair dryers and speeding stage coaches at the pass.

Anyone who would use a hairdryer while in a bath should feel comfortable playing Russian roulette. Anyone allowing another person to use a hairdryer while standing above them while they are in a bath should be comfortable being a machete throwers target. 

2. Yes: Despite the forgoing, it should be able to be made ""reasonably" [tm] safe. 

An RCD will work.   You say that , "residual current will not help - current through phase and neutral can be equal." but this is extremely unlikely to be true - and it it as true there would be no danger. Current "enters" via one mains lead and "returns" via the other. If entering and returning current are balanced then, whether immersed in air or water, no current flow exists to cause problems for the user. Water is a moderately good insulator but when contaminants are added it can conduct quit well. If the bath has a path to earth - either due to an earthed  metal plug connection or an informal path, then if more than a few mA of current flows to ground an RCD (residual current device) would trip. 

Adding an earthed metal plate or body to a hairdryer will make an RCD work better. Most hairdryers and similar are purposefully not equipped with an earth connection but, as the challenge is to make the appliance "bath safe", provision of a generous earth framework would provide an immediate earth path for current when dropped in water and trip an RCD very promptly.This "tripping plate" could be entirely within the hairdryer and insualted from user contact but designed to be sure to be in contact with water if the appliance was dropped in water.

An isolating transformer will increase safety BUT has a 'gotcha' mode. If the voltage is not ground referenced then there can be no current flow from mains through water and user to ground. But there can still notionally be flow from eg phase, through water back to neutral. The voltage drop across the water would also become voltage drop across the user. It is not obvious how much current would flow via this essentially high resistance path but quite likely not much. As under 10 mA can be fatal, there may be enough. In this case, use of an isolating transformer would prevent an RCD operating and would increase the chance of user shock. It would be easy enough to measure how pronounced this out and back again flow was likely to be.