I actually used to fill the handlebars on my 97 CR250 and 98 CR125 with spray foam. But that was cause those thing vibrated so bad. I don't think it helped one bit, other than maybe a placebo affect. You could do the same, there would be no harm in it, but you'd probably only get the placebo effect. Just make sure the hand warmer max temp isn't higher than what the spray foam is rated to.