There would probably have to be another voltage regulator in between
the alternator output (from it's standard regulator) and the inverter.

I bought one of those little inverters that go in a cigarette lighter
so I could charge extra cell phone batteries in my car. I have an
unswitched outlet that is on all the time and the charger works great
in that. As soon as I turn the engine on however, the light on the
charger blinks rapidly back and forth from green to red. (charging to
charged). I'm not sure if it's charging but I haven't wanted to risk
the $70 battery and $40 charger to find out.

Sounds like you are pointing me towards similar problems on a much
bigger and more expensive scale.

BTW my AGM's are working great the way I am using them, short motoring
periods and an overnight plug in about once a week to top them up.
With a 20 HP engine and very minimal hotel loads, they don't work hard
anyway. I'm just wondering how they would like being driven by the
low tech Hitachi alternator with its tiny built in regulator if I had
occasion to motor for 12 - 25 hours straight to move the boat
somewhere.

--

Roger Long