"FM Expansion I/O for System ON
I believe output #2 of the Expansion Module (EM) PCIOM is always mapped to System On. You can wire this to an input on the Main Module's (MM) PCIOM, then map that input to the output that you have the MM's System On mapped to. This should assert that output if either the MM or the EM is shut off.
I haven't tried this myself, so you may want to test it out on a couple of FM simulators before you give it to the customer.
Verified on Simulator with 6.21.0 FW"