I would advise against brewing in the Kelly or Thermette. The Kelly would likely hold the grounds quite well in the tight areas of the chimney and there is definately nothing worse than stale coffee grounds to ruin your next boilup. I imagine the Thermette with its very restricted spout would even be more difficult to clean of grounds.

I just take a top-of-cup filter device, brew coffee in an cup with integral press arrangement or for my disaster kit filter brew into a Nissan thermos with cone. This way the water can be used for soups, tea, coffee, hot chocolate etc in a nearly endless manner.

Rancid coffee oils in my boiler would be something I would not want to have to figure out how to deal with.

BTW...thanks for the image compliment. Only the Kelly Kettle image is mine....the Thermette image came from their site.