i guess youd be right about the hysterisis. its not needed since the light is already controlled.
i have a 99 ranger, and i would have to imagine the alt light is based off the ecu- since everything else is. i guess one way to tell would be to bring up my scanXL software and see if i can see charging status on it. but either way, a mosfet on the charge light like you said should work for anything.