How about an electric baseboard heater set to 45 or 50 degrees? No need for the er "heated seat:" WVBill was kind enough to point out.

Another option is to let it freeze, but to put an "expansion joint" in the bowl & tank. A small 16 oz plastic soda bottle with a little wieght in it so it is 75% submerged. When the water freezes, the bottle will crush & hopefully the commode won't crack. This type of system is used in pool skimmers to winterize them. The bigger bottle you can fit in there, the deeper freeze it will hold up to.