Really? I've already tried heating the sleeve with the small cap off and I don't notice any difference either way.
I also like the idea of using the Fleshlight while it's wrapped around a heating pad. It works good in theory, but it's a bit of a hassle to use both at the same time. For one thing, the heating bad is too thick and big and adds too much weight to the case. Plus, you have to worry about the long cord and the case slipping on and off during strokes. I wish there was a heating pad that would be the right size and thin enough to wrap the case without adding too much thickness and weight to it.
So far the only thing that comes close to what I just descried are Heating Socks:
They're light-weight, battery-operated, and can be stretched to conform to the shape of the Fleshlight case. The downside is the heating element is only located on the feet back of the sock. I doubt it would be able to heat the Fleshlight evenly.