How big are the hydraulic accumulators? How many gallons will they hold?
The problem with the container is that you've got to put feed water into it or you'll have a short run. It takes at least a gallon/minute to get any useful work out of steam power. If the boiler is pressurized to say 50psi (keeping it relatively safe) then I also need a 50+psi pump to inject the feedwater into the boiler. Can I assume the freshwater lines in the plane are pressurized to at least 40? Perhaps that could be how I get to at least 40psi.
It all suggests the minimum size for a boiler to produce 500W-1KW is going to be around 60G or so. Again, I haven't worked it out, but I've played with some steam calculators.
Steam Calculators: Steam System Modeler