Mr Guppy is unbelievably ignorant of just how lucky he is in FAAland. If I want to fit a Tanis heater to my UK engine - totally independent of any aircraft systems - I have to submit a mod including the the full spec of the ins and outs of a ducks backside, a blank cheque and an open-ended completion date expectation to EASA for a total benefit of perhaps 14 days extra flying days per year, 13 of which I cant get to the airfield which wont be open anyway.
Ignorant? I spend plenty of time in EASA-land, as well as numerous other jurisdictions, too. You're correct: my maintenance qualifications are FAA certification, and I'm quite content to operate within those bounds.
You do not require a field approval, however, to operate a combustion heater, which will easily fit in the trunk (or "boot") of your car.
There's more than one way to skin a cat, more than one way to wet a duck (and it's backside), and more than one way to preheat an engine. The manufacturer recommended method involves a heated hangar. In lieu thereof, other methods are available.
This is an interesting product.
Reiff makes an excellent product.