My concerns would relate to those already listed.
Fooling around with exhaust is not worth the possibility of something going wrong? As mentioned; a head gasket starts leaking? It wouldn't take long to get sick from carbon monoxide fumes, & you wouldn't know it at the time.
IMO! Dress properly.
Buy a pair if Carhart overalls & be done with it. Wear a full face covering for the your head. I'm toasty as can be as I was plowing yesterday except for my hands. Looking into investing in battery operated gloves for a little heat. Old injuries, broken knuckles, fingers are taking their toll in blood circulation as I get older & older.
NIK,
Or? Look into some ice fishing shanty heat? They've got small one man heaters for dem'dere shanty s that work quite well. Small propane bottles that last at least 2/3 hrs..???????