As you suggested, the gun should cool. I leave mine in the garage the night before. In the morning I will snap off a cap or two before loading to dry out the chamber. (because thats how I was taught many years ago.) As far as the powder goes, some times I leave the bag with the gun in the garage and sometimes I leave it inside. I have never had a load not go off weather it was just in for a day or 4 days in either the in-line or flint lock so in my experience, warm or cold powder does not matter. I feel there is to much steel around the 90-100 gr. to cause condensation and that the powder cools with no problems.
Keep it all cold and you'll be fine.
Now. Keeping rain and snow out of the pan on a flint lock... That is a whole nother matter