You're correct about efficiency, who cares in the case you describe.
You're only out cost, is the cost of your inverter a better trade off than a thicker wire?
I'm not sure about the "more stable power" statement. Modern cars have so many low voltage processors and associated circuitry the power needs to be well behaved at the alternator (by that I mean no overshoot when going from heavy to light loads and back).
But I agree with you that your system can provide absolute control over the charging voltage. If using LiFePO4, would be a good advantage.
No way would I put the inverter in the engine compartment! Unless all the components inside the inverter are auto grade certified for 125C or 150C. Would need to search for an "underhood" rated inverter, which if they exist will be pricey.
Dave
