Green coffee is normally 10-12% moisture (older beans might be at the lower end of that spectrum). I seem to remember from my homebrewing days that most grains are around the same percentage... Thus, I think itll work very nicely!
However, green beans have lots more caffeine in them than roasted beans, and its possible that this could leach out... ? Might not be the most relaxing of heat packs!

Cheers
Stuart

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