I can't immigine there would be a safety concern with that. I have run my block warmer off of my generator and off of my inverter at various times without any problems. The wires going to the block warmer should not actually come in contact with metal on the bus.
It's not much different than setting a cast iron pot on top of an electric stove. The pan gets hot, but there's not electric current passing through the pot.
Quis custodiet ipsos custodes (who will watch the watchmen?)