> Most people's answer is going to be along the lines of "sure I'll take a free battery but if the hose barb is functionally equivalent and gets me my car a week sooner than it'll depend on how badly I want the car back"

No. That solution might be acceptable if the alternative is spending 15k$, but if money is not a factor, I bet anyone would take a manufacturer approved replacement over "some screwed in thingy that's probably within temperature range" just to get out of your rental a week early.