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Problem is going to be weight, takes a lot of mass to hold fairly few BTU. Water holds 1 btu per lb per degree F temperature change. To hold 1 gallon worth of LP BTU's (91,500 btu/gallon) assuming a 140(70-210) degree temperature difference in the water/antifreeze you'd need 657 lbs of water (79 gallons). My furnace is 80,000 btu so that would be 68 minutes or run time. I think my last LP fill was $1.25 a gallon but has been over $3 in the past.
So I'd need 2 (40) gallon hot water heaters sized tanks, an heat exchanger in the exhaust (wrap exhaust pipe with copper tube and insulate), an expansion tank and a circulating pump.
Then a way to put the heat into house (already have some radiant hot water floor heat in part of my house so that part would be easy for me. Park in the garage, hook up 2 hoses and turn on the 12v circ pump.
I don't know the specific heat of brine or how that would change things.
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