So after much reading and research into the whole "backup" BEC saga. I think I've found a solution!
My first idea was just to run a 5.5v backup lipo in parallel with the 5v UBEC output.
This idea has been vitoed by most. Too many unknowns apparently.
The other was to parallel all the BECs up, but this would cause harmonics due to all the square wave interaction.
So I figured we cant mix power supplies. Other than using a microproccessor controlled mosfet to switch
power supplies when and if the bec dies.
We use a relay :) Bear with me.
The main power supply powers the relay. With the relay closed (NO) power is routed via the bec to the apm.
If the bec fails, the power to the relay fails and is switched to the auxillary power supply (battery or another bec)
This completely seperates the two power sources.
Will the relay be able to switch fast enough to the auxillary power source without the APM losing power?
If not, im sure a power capacitor will keep the APM alive long enough for the bi-pass surgery to take effect?
Above is a recording of the ciruit just in case my explaining skills are dodgy :)