One item that might be a bit safer than hand warmers for frogs (they can cook a frog with a quickness) is Phase 22 packs and panels. These pack contain a liquid that is designed to keep a closed in area around 71 degrees. I use them for shipping frogs, but I imagine, in a pinch, they could be used to keep temperatures at an acceptable level, at least for a while. Hopefully long enough for the power to come back on.

For more info, do a search for phase 22 packs on some other forums ( i dont wanna plug any specific forums). They are sold here:

http://www.alphaprobreeders.com/products/Phase-22-gel-pouch.html

http://www.joshsfrogs.com/shipping-supplies/phase-22-panels/phase-22-panel.html

That being said, if you have gas powered stoves, a hot water bottles might suffice if used carefully.