If your goal is to waste 50 watts worth of steam, your best bet is probably to PWM a resistor. If, however, your actual goal is to avoid letting out lots of steam when there isn't a huge demand for electricity, then what you really need is a switching power supply whose feedback loop operates effectively backward from a more typical one, such that it will draw maximum current under conditions of minimum load. Because of the odd feedback response in the system, it will almost certainly be necessary to use a two-stage design: stage one will use the generator to charge a cap whose voltage may vary widely (probably by a factor of at least 2:1). Stage two will feed that cap into a more conventional switcher to yield a regulated output. Note that if the load can quickly increase from e.g. 1 watt to 50 watts, it may be necessary for the first stage of the supply to shut down for a significant fraction of a second while the generator speeds up enough to supply 50 watts; the cap must store enough energy to deal with that. Note that trying to draw current from the generator when it's significantly below the necessary speed will increase the amount of time the cap will have to supply power, and in most cases increase the total energy the cap has to supply.
BTW, a simple PWM/resistor approach is shown here. Not much to it. The variable voltage, series resistor, and cap are used to model the generator (here, scaled to produce 10-75mA). When less current is drawn than what's needed to maintain equilibrium, the generator will speed up and the voltage will increase; when more current is drawn, it will decrease. The latch will output high when the input is 2.5 volts or above; since I'm using a 4:1 voltage divider, it will detect whether the cap is above or below ten volts. When the cap is above ten volts, the transistor switches in a resistor that poses roughly a 100mA load.
Note that changing the input current to this circuit (meaning the difference between the current that must be drawn from the generator, and the current that other stuff uses) will only marginally affect the voltage on the 1uF cap (meaning it will only marginally affect motor speed). In practice, generator speed variations would be nowhere near as severe as shown here (the generator's inertia would behave like a much larger cap) but I chose a small cap so as to show how the PWM works.