Not a jacket yet but kinda backwarmer.
I have tested this now couple days (in the mornings, daytime is too warm) and it is absolutely marvelous.
Two 2.1meter (210cm) loops of resistance wire connected parallel (about 27ohms each), power directly from bike. That makes two ~7watt heating elements.
Heats to 34C.
Works ok if temperature is around +3C as long as speed does not go over 80km/h.
Over 80 it does not feel warm, I need to cut 10-20cm off from wires. That makes it run a little warmer, around 40C-50C max. Should be enough and it will also prevent burning if controller fails and connects battery straight to jacket.