Correct me if I am wrong, but I think if we did it that way it would scale a higher voltage to be a higher current... wouldn't it? High reference voltage = high current.
I was just thinking we would use an Op-Amp based inverting amplifier, then the reference/current limit would go
down as the DUT voltage went up. This will keep the max watts the exact same over any range as long as the correct scaling factor was used. It should be an obvious linear relationship, as Watts = Volts * Amps.
But as Ronv just pointed out, if the DUT goes nuts, drops to a really low voltage, the system might see that and say... MORE CURRENT NOW. So this could lead to problems. First it could blow fuses for no good reason. Second, it could make the system not want to start by holding the FETs short. It's a simple problem to solve though, we just need to make the output of the scaling circuit cap off at some reasonable limit, like with a zener or something. This way, the current could never get over say... ~50 Amps no matter how low the DUT voltage was. Should be easy as pie.
Flow sensor.
I more or less like the idea of a water flow sensor, so long as the threshold is really low. Though, for use as a simple safety and not an actual analytically measurement system, it is easy enough to just make a flow sensor. All you need to do is glue a thermal sensor to a 1/4 watt resistor, putting just enough regulated power through the resistor to make it heat up a good amount, then measure the temp via the sensor. As the flow increases, the cooling will increase and the temp will decrease. If it is installed at the water inlet, it will only be affected by the water temp. If it's installed at the exhaust end, then it can be made to take into account the system output temperature as an added bonus. With water we may have to cover it in something slightly insulating like plastic, but over all this method works well.
Edit: Also note, that the above is talking about something that is almost no different from simply slapping a thermal sensor on the pipe. If it gets over a certain temp, it's almost certain that the water is not flowing.
In an automobile
mass air flow sensor, they have a small length of thin resistance wire that they put a regulated current through, then periodically measure the resistance of the wire to infer air flow. Or instead, as stated in the wiki article, they will measure the current going through it when it reaches thermal equilibrium. Fairly straightforward system.