Well does the oven go as low as 150 deg. F ?
250 F would be hot enough to steam the bag. If the foil is sealed around the rice bag then the moisture would be retained and less likely to scorch the cloth.
I have no idea at what temperature cloth (or rice) would ignite. I think I remember paper (like books) burn at 451 F (If you can trust the book title).
Can you moisten the rice bag before freezing or heating?
Any special preparation of the bag used to enclose the rice? Or preparation of the rice?
Oh, why not put it in a ziplock or other self sealing bag and warm it in a pot of water on the cook top? Then you can remove it from the heat when the water is very warm but not yet scalding temp (102 to 108 F)
*the human pain threshold is around 106-108░ F