>>"She also insists on a hot drink before bed -- at least one liter."
i don't mean to get into more personal areas here, and there's no need to reply, but your wife must have one HUGE bladder. The nerves in a normal human bladder begin sending signals to the brain when it's about half full. I think i read somewhere (Artic 1000 website maybe???) that you're also a Bio Prof, besides teaching Math at the University level, so, i'm sure that you already know this.
since fuel is being used to heat the water for the drink anyways, why not just heat the water, place it in a Platy, wrap it in a pack towel to insulate it a bit, and place it "hot water bottle style" in the bag.
40min to a couple of hours after consumption at the most, i would guess, based upon my physiology and aged, somewhat non-elastic bladder (sorry for the unsolicited personal info, but i don't think my bladder could even hold 1L), and i don't mean to project such things upon your wife's physiology, i'm going to be drainin' the main, so to speak.
this would involve, waking from the warm, safe "Land of the Sugar Plum Fairy", getting out of the bag (particularly in a female's case), out of the shelter, and entering the cold, cruel world of reality, becoming a bit chilled, returning to the bag, and attempting, without the aid of that hot 1L beverage this time, reheating the cold air in my bag.
in fact, given my age, 1L at bed time will probably do me good for two 'pitstops', so to speak, b/f sunrise.
During the Star Trek Next Generarion years, i once was heard mumbling in the Men's room at work as i stood there ~40min after drinking a cup of hot Earl Grey Tea, "Tea, Earl Grey, body temp" - other STNG fans were immediately identifiable by the raucous laughter which filled the Men's room. I think you get my meaning.