Alex, I saw a demo somewhere, I dont recall if it was TV, online, or real life, of a guy who boiled water in his hat by doing that. It looks like a great idea, until you have to drink out of your hat...and the hat didnt hold that much. That being said....
I carry a NATO canteen with tin cup, and, if needed, can boil water in it. The easiest way to do that is by heating rocks & dropping them into the water-of course, you need a steady supply of rocks, but a few minutes of fire prep, with the intent to do this, solves that. The bonus is the handle wont scald your hand