I heated a front rim last fall so much that it warped under the braking pressure. I still have the wheel, maybe I should take a picture. The braking surface is concave in one spot. I noticed it when my braking started acting odd, pulsating each revolution.
This happened because I was going downhill at over 30mph on the ebike and only my front brake worked, and the light turned red on me at the last second. With two good working v-brakes, I would say it would take a massive hill/fast stop to heat them that much to warp. It was a narrow older mtb rim too.