Well when the ac is there
 

Well when the ac is there the wheel is still spining. If the ac is removed there is one less wheel to spin. Is it would help a tiny bit to remove the ac pully vs. letting the wheel spin freely.

ON Off
 

Nope you need to regulate about 5 amps or less that feeds the field winding unfortunately the connection in the new alternators are all internal so getting to the field wire is a bit tricky.

Re: Aren't most new alternators
 

just checked: 1 big, 2 small. later i'll have a look in my service manual and see what it says about the 2 small ones.

wires
 

Probably a remote sensor for voltage regulation and the warning light. With any luck the remote sensor wire can be used to reduce alternator output by increasing the voltage to it to shut down the alternator or at least connect it closer to the alternator output giving it a higher voltage than the loaded side of the battery positive terminal with the line drop from the alternator power wire run.

Another way to reduce alternator losses
 

You could rig up an air cylinder connected to the intake manifold as a belt tensioner:

At full throttle, the vacuum drops to near zero, the air cylinder releases tension on the belt. This stops the alternator and belt.

Under decelleration or at idle, when vacuum is high, the cylinder pulls in the belt.

This wouls eliminate both belt losses AND alternator losses.

would work
 

sounds great but have you ever seen flying belts?? and the poor regulator if external would go crazy with field current trying to output some voltage when the belt was off. Internal regulator would not be a problem as they operate off the generated voltage inside the alternator.

Most alternator belts I've
 

Most alternator belts I've seen also run the water pump. Also, I think the belt/pulley combo isn't a high reliability kind of clutch, hence it's restriction to things like small lawn tractors.

But hey, that's thinking outside the box. I like that. 8)