Hi Neil;
It is probably too late for you to consider this in your design however I'll throw it out there anyways.
A few years ago I modified my RD-1B to allow the installation of a stator & rotor to have a back-up PM type of generator between the engine and PSRU.
I've got a few hours of ground running on this and it has run flawlessly. It was a big job to retrofit it into my existing PSRU, but possibly not be a big deal for you to add as an option on your unit.
The full write up on how I did it along with parts etc. can be read here in a PDF document.
It might be worth considering, as I know that if I was in the market for a PSRU, I'd certainly pay extra to have a built-in back-up power source.
Todd