Both Rex and Philip are spot on.
Your 26800 mAH battery stores about 26.8 amp-hours at 12V potential. Assuming one could draw all 26.8 amp-hours, one would have about 1.16 MJ of energy available.
But that’s not very much energy. The specific energy of wood, for example, is about 15 MJ/kg. So, a 75 gram (3 oz) stick has about as much useful energy as your battery (as it pertains to heating–don’t run outside and try to plug a stick into your cellphone to charge it).
Another caveat is the limited power distribution over USB. I thought it was limited to ten-ish watts, but a quick Google informed me that 100W is possible. So, you could run a 100W heater, but remember, you’ve only got 26.8 amp hours available, at 12V. Ignoring the inefficiencies of the power electronics that would be required, you’d only be able to run your 100W heater for a little over 3 hours, assuming you took your battery from full charge to zero.
So, a 100W heater, for about 3 hours. 100W is pretty small for a space heater.
However, a minor tangent: I grew up in WI, and we always used electric blankets in the winter months (August through June). It seems like one could design a relatively lightweight, small electric blanket to go in your bag with you. This could draw USB levels of power, and produce a noticeable amount of heat, since the heat is produced within your down cocoon. (edit to add: Rex basically mentioned this above with his wife’s hand warmers).
That’s mostly a thought experiment; the practicality of doing so based on the premise that you have extra, “free” electrical energy available–you are already carrying the charger and battery. Otherwise, it would be undoubtedly more efficient (in terms of weight carried) to simply bring a bag with another ounce or two of down.
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