how about temperature hiit then?
for 30mins, have temp at 50degrees, no sweater
for 30 mins, put on sweater and turn up heat to 80
surely this would create the desired metabolic disturbance?

Body wouldn't have time to slow down metabolism to survive, and we'd still benefit from the metabolism boost to keep body warm?
to crank it up, do jumping jacks or something to keep warm while freezing?