Thinking this over, the best place to seal the hotend is between the throat tube and nozzle. What are the advantages/disadvantages of an integrated nozzle and throat to make this seal? Does a throat with a Teflon tube improve this seal? Is it reasonable to cut a gasket out of 3mm ID 5mm OD Teflon tubing to make the seal for M6 thread parts? I intend these questions to all address the main question to prevent a leaky hot end.
A seal between the heat break and nozzle should be achieved by tightening them while the hotend is hot. After assembling the (cold) hotend, heat it up to somewhat above the highest temperature you will print at and then tighten the nozzle some more. When it cools down again, you will have a very tight seal. If the nozzle still leaks after this tightening then it is defective.
Using teflon is possible but will limit the maximum temperature you can use your hotend at safely and you won't be able to print some higher temperature materials.