Hi D,
I would think that would be a question for your Veterinary Oncologist. In researching- There is such a thing as Hyperthermal treatment of tumors done DIRECTLY to the tumor. I would think if this is not clinical heating, rather heating for general body warmth , there would be absolutely No effect on the tumor itself.
Keeping kitty comfortable with a warm heating pad sounds like a good idea to me, just make sure in addition to the LOW setting you do not place your kit Directly on the pad, rather a towel be placed between the pad and your baby.
Let us know what you find out, so we can help someone else with a question like this.
CML