Forgive me If I get the wrong end of the stick.
are you trying to get the tea as hot as possible & keep it that way, or is it just required to get the water over 100degC & then make the tea at a colder temp.
Because if a hot cup of tea is required surely decending to brew, would be useless, because as soon as you climb again it'll all boil away.
Otherwise why don't you just boil the water on the ground & then reheat in the Air.
It's gonna keep me away all night now!