Well, for one they might have a problem with the pressure. Come to
think of it, a car radiator- typically kept below 15 psig by protective
cap- would too. Unless either would be used in a closed-loop with a
heat-exchanger in the path of the domestic hot water. Which should be
rated for the pressure differential- sometimes 80 psig. But then,
where to put that exchanger?

With closed-loop, any worry of leached lead from rad would be nil.

Intriguing idea.

HTH,
J