Seems like asking for broken stuff in an mtb environment, but I don't see why they wouldn't work.
I've got a few friends that put the smallest "chemical" heater packs on top of their toes for winter mtb, and they swear by this.
I go with thick socks and some marginally effective half-covers on the shoes. I've got full covers that are really warm, but I always seem to avoid the hassle of stretching them on, and zipping them up, and instead go with some winter shoes by Diadora. Just warm enough, without seeming like a big, clunky boot.