I hope you guys don't mind me chiming in on this one. I've been curious about leather mugs and cups for awhile myself, and I'm sure I'll get around to making one in the near future. The link that cem listed for the product called "ep21lv" is rated as a service temp of 250F. Since water boils under normal pressure (sea level) at 212F, this seems like it should be good for hot drinks. I just wouldn't use an instant coffee and put it in the microwave to boil
The application guide shows that it cures in 24 - 48 hours at room temperature, which sounds reasonable to me, but its cured color is clear-amber. Most likely will change the tint of your project a little.
I'm certainly going to give douglais's suggestion a try, as I don't drink warm liquids, but I may contact Master Bond if I change my mind. If anyone does contact them, let us know what they say and the amount and pricing they are willing to supply.
Thanks,
Ryan
P.S. After another minute of looking around, I realized the link that rtp41 was talking about was referring to EP30HT. This one can withstand temps up to 400F and cures in 24 hrs at room temp.