Have you thought about how your engineer is going to do the fabrication of turning your apu into a steam engine?
You may have a box of hand tools on board that belongs to your engineer. But to fabricate and connect things like pulleys and gears onto axles you're going to need to be able to weld. I've come up with an idea for this. You could use the aircraft main battery as a supply and the steel spring rods inside the passenger seats as the welding rods. Aircraft battery is a Nicad 24V with lots of juice, but you're going to have to watch the temperature during welding or it will explode! Then you will need to recharge it again.
The engine FADEC control alternators are your best bet. However they are bolted on to the front face of the engine gearbox at the bottom of the engine, which I imagine will be embedded in the beach! These little alternators produce quite a lot of power for their size and can power the entire engine at only 7% N2 (don't ask me what RPM the alternator is running at, I have no idea) They have two output windings, one for each FADEC channel. The output from these alternators is AC, so you're going to have to run it through a rectifier to be able to charge your battery. Like the standby TRU.
Well you did say you wanted McGyver!