I use newspaper in the bottom of all my tanks with the substrate on top of that.... Paper is made from wood, substrate is made from wood. They're both equal "fire hazards".
Personally I've never heard of a reptile heat pad causing a fire (especially through the glass). I have however seen a massive, deep-tissue burn on the belly of a ball python that was allowed to bask directly on hot glass above a heat pad and it took several months to heal.
Many breeders use only newspaper as substrate, so unless your heat pad is INSIDE the tank (bad idea) and the wire is all chewed up and exposed (again bad idea), then newspaper will be fine underneath your normal substrate.
~kasey