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