the custom kydex sheaths are wide and flat because they are 2 sheets riveted together. That means you're going to wear it behind the point of the hip. Combine that with the their high riding style and it would be difficult to get to that knife while wearing a parka.
Not always: custom Kydex sheaths are...er... Custom. They can be different widths, thicknesses, and different attachments. I strongly recommend the Tek-Lok from BladeTech. Many custom sheath makers will use them if you ask. Ask that they be drilled to use the Tek-Lok in all three orientations.
On my heaviest coat I have a strong flap (2" x 3") to which I can clip my sheath/Tek-Lok and the knife can ride just under my left armhole. But I rarely find that necessary.
Firefighters be warned, however: Kydex sheath will soften quickly at 250-300 degrees, and I don't know what the ambient temps. near a fire would be. Leather may be a better choice after all.