Hot water bottle / wheat pack

yep. I was advised to use one on my pelvis when that was sore. You have LOTS of insulation between the outside and your bubs. I imagine if it was hot enough to affect bubs (plural :) ) you'd be dealing with a few problems of your own ...just use common sense.

I think it only gets dangerous if you raise your core body temp excessively

Yep, definitely. And as you get further along, you'll be amused at the kicking and shoving your bubs will give it, lol!