If the Life Safety System ( Fire Alarm ) is up to snuff a duct detector is going to shut things down well enough in time.
Not quite the same set-up...
But I rigged a 45kVA XFRMR, dry-type, above a grid to give isolated power to data loads. Upon the detection of fire, within this space only, a shunt-trip killed total power - with a time delay of seconds so that the data state could be frozen/ preserved. Subsequently the dry fire system would be flooded into the now unpowered space.
So the question of plenum return air really would seem to be a life safety question not to be answered within our Code.