Never tried it directly between the hammock layer and UQ, but I'm sure it would help. It does seem like a bit of a pricey way to add heat over time, but may be good in a pinch. My only concern would be that they might be a bit tough to strategically place on where you're coldest (feet, etc.).
Personally, I much prefer the hot water bottle trick. Boil water, put in a nice nalgene, wrap the nalgene in something like a thin fleece (I've created a thin fleece sock for it) and enjoy the warm goodness all night. It allows me to easily move it around to the feet, to an armpit, to hug it or to set it aside if I get too hot. And, in the morning, I can drink the warm water or reheat for breakfast. No need to use and throw away the chemical packs.
Father. Husband. Scoutmaster.