When I got back home I decided to see if my theory on the voltage being supplied to the engine's electronics from chopped wave transformers was correct. As it turns out, it was. I explain in detail HERE:
I've also been tracing out the boards in a LionChief engine, and while I'm not nearly done I've got enough of it mapped out to be very suspicious of the claim that more than 18 volts can damage them. So far it looks like they should be able to handle up to 24VAC with out any problems. Of course I recommend following the manufacturer's instructions, but to come full circle, I am in serious doubt that over-voltage caused damage to Denny's engine.
JGL