I would say a no go on the heating pad, purely due to the fact that the heat will not be distributed evenly and there would be no humidity for the eggs and this would cause them to dry out slowly.
Great news! We had a client recently with the same dilemma, as we have quite frequent power outages in Africa, but we have found in the end it is about how well the machine is sealed. We must also remember that in nature the mother hen does not always sit on the eggs and nature has designed them...