Bill, I really think the only way you're going to achieve what you want is with a self referencing PWM like dk said. That's the only way you can get the full 20amps all the time and compensate for increased heat (and load) in the electrolyte.
Or if you want to try and you know how the increased amperage requirements line up with temp of the solution... You could throw an old car water temps sensor in your cell and based on it's output have it adjust the power to the device.
That is, instead of turning this 'knob' and trimming a pot to adjust for increased temperature, if you use the right thermosistor with the right ramp up, you can hook this inline directly with your knob and at cold it will have some high ohms and as it warms it decreases the ohms just as you do with your knob.
I don't know if trying to regulate it by any other means than the concentration of electrolyte in the solution woud be self defeating. A breaker or thermal regulator will shut off the cell from time to time (not desired operation). Any type of thermally activated voltage regulator (reducing voltage as cell heats up) would not be a good idea either. My theory is that as you reduce the voltage available to the cell, its amperage requirements will increase, defeating the purpose of reducing the voltage. Again we're back at keeping an eye on the electrolyte concentration, and put a fuse or breaker in to shut it down in the rare instance that it draws too much.
I don't hink everyone is looking at this correctly.
The only way to get true gas savings is to completely remove the system from your cars electrical system and run it from a marine deep cycle battery recharged by a solar panel. This way you are not using your cars alternator to produce the hydrogen. This type of battery will run a hydrogen generator for a long time on just 20 amps and when the car is not in use the solar panel will recharge the battery. This will put to rest all the skeptics who say you can not produce enough hydrogen for the amount of energy you get out of it.
Once the initial cost of the battery and solar panel are payed for by the savings in gasoline you will be getting free energy to produce your hydrogen.