This sounds like a dumb question, but do you suppose chemical packs like these could burst and let out the hot messy fluid in jumps?
Littlerain, there are re-usable toe warmers, which you warm up in the microwave. The good ones use a phase-change to store extra heat. But they are all pretty thick, and would totally mess up the fit of the boot.
Maybe these full-insole warmers Isk8NYC mentioned are thinner than the similar toe-warmers, and would therefore interfere less with fit.
A few people order custom fit boots allowing space for warmers. When you don't need a warmer, I guess you use an old used warmer to take up the same space, or some other extra insole.
I sometimes warm up my boots (e.g., with a hot air drier) before putting them on - but that doesn't last 9 hours.