I'm thinking maybe feed the field winding with a variable output buck
regulator and include the battery in the voltage control loop.

What do you think?


Rectifiers aren't in alternator body? Is the output winding three
phase or single phase?

Its a 3-PH alternator with the rotor bolted on the end of the crankshaft -
the rectifier/regulator pack is hidden under the tank somewhere.


Linear produces heat/waste. You want to be switching the field on and
off... PWM.

There's no shielding of the wiring on the motorcycle like you'd have with a
car body - pulsing the field might even cause an RFI nuisance.


Bistable works just fine, you're just more likely to have radio
"whine" that varies in pitch with RPM.

I don't have a radio - but I pass plenty of houses that do.