This is more of a technical overkill solution, working in a fixed indoor space, probably not in the immediate realm, but imaginable. It solves the issue without touching the hands or having any interference whatsoever. It is an invisible solution, aka, least satisfying design solution.
Combining a few known systems:
A thermal imaging system.
Video recognition software that recognizes hands specifically and can provide the target data of the sub temperature areas.
A set or series, of low wattage infrared lasers with scanning-positioning mirrors mounted strategically in the local environment, or in an enlightened public workplace.
Processing and software to direct the infrared lasers and apply a measured amount of energy to required areas of users hands, and measure the result in a feedback loop.
Precise, variable, non-contact, invisible infrared heating.
Other adaptations would be for individual users to be identifiable, allowing the ability to personally dial in warmer or cooler operating temperatures. This system would be ideal for a country like Japan, where the post-Fukoshima situation has required offices to dial back heat settings to save energy.