That is an interesting question.
Myself, I bought a re-pro set from a member on here - but he has stopped making them. I haven't seen any other sources for these in quite a while.
Regardless, you'll get more responses by posting this question on the SOHC/4 Bikes forum, as we all use insulators (different part numbers, but the same materials).
The insulators do see a good deal of heat, but I don't personally know a value. Wikipedia indicates that gasoline boils at high as 395 degrees F - and I know that the fuel in my carb doesn't boil. So, using that as a guide, I'd say that any sealant good at 500 degrees should be good guess.
As you may know, finding these insulators can be tough, especially in good condition. If you do find a set (without leaks), there is quite a bit of discussion (in the SOHC/4 Bikes) forum about what chemicals can be used to soften the old, hard rubber.
If you either can't find replacements, or are too impatient, I would personally try some sealant (like Permatex 1372 - rated for up to 600 degrees) and a tape overlay. I couldn't find any black tape rated for over 200 degrees. Lot's of silver tapes, though (flue tapes). Being cheap, I'd probably try to use regular 3M 33+ electrical tape, and see if it stays in place.