Finally got through to FiTech tech line, they said ECU needs 12V, lower won't work.
When I explained problem with Mopar ignition not having these 2 simultaneously, his response was , "you need to find a source".

Sounds like John's relay may do the trick.
Only concern, if the ECU voltage is so critical, what about the relay switching time between engine crank and run?
Won't there be a few msec of dead time as the relay switches between the 2 sources?