back in the day when i used stock bags, i used a simple nylon strap around the bag and mounting brackets as a backup. these were the old-style krausers with the loop frame/mount.
not sure if this will work with the newer bags & mounts as it might be possible for the bag to drop onto the rear wheel?
nowadays i use jesse boxes and their mounting system is nuclear strong... the only way it will fall off is if you forget to close the latch that holds the bag to the mount.
ps => bob - are you going to write an article for the ON on this? it sounds like a great subject.