The question also has potential application to the ForkenSwift. Some EVers use "switchable" alternators as a basic form of regen, typically driving it off the tailshaft of the motor to recharge the house battery.
Apparently you can also mod a standard 12v alternator's voltage regulator to put out much higher voltages, so you can use it to feed juice to the traction pack.
With any luck you may have a remote voltage sense to monitoring the voltage at the battery that you could play with but generally the regulator is built into the alternator. The only problem with disconnecting the brush is that the drive circuit may go a little crazy trying to drive the field (which is disconnected) harder and burn something out in the process (remote chance however) or may spike pretty hard when you turn it back on.
I still think the best way to do it is to add a separate battery to charge the car battery above the voltage that the alternator regulates to and keep the car battery fully charge and essentially "off line" and run on a second battery powered charger / supply.
Actually you could isolate all the battery loads to the second battery except for the alternator and starter motor and even kick in the alternator to the second battery when slowing down / braking using the brake light to switch the alternator output to the second battery to charge it. . . . This actually sounds like a pretty good idea using a deep cycle battery that could be grid charged etc. and it would not require high current output for the starter motor since the original battery would be doing that.
Actually you could isolate all the battery loads to the second battery except for the alternator and starter motor and even kick in the alternator to the second battery when slowing down / braking using the brake light to switch the alternator output to the second battery to charge it.
Be careful with this idea: alternators have fairly dumb control circuits. They don't limit current and will happily burn up the alternator/boil off the battery trying to charge a dead battery. You can limit current by having a smaller charge wire to the battery, e.g. 20 feet of 12 gauge wire.
I don't think you'll have any electrical problems putting a switch on the field. It's not unusual to go all the way on, and the switch would take any turn-off voltage spike.
I think the biggest problem is the fan. They're sized for high draw at idle, and no doubt draw a lot of air at speed. You do need some air, but it could be a lot smaller as long as you turn it off at idle (assuming you idle at all...).
The field voltage is supplied from the inner diodes of the alternator so if you connect it to a low voltage run down battery it loads the output down and the voltage available to the field as well so it limits the output that way and it can only generate so much current anyway unless you really start spinning it really fast and that takes more power and belt load and would probably cause slipping of the belt at some point. Another option is to Diode isolate the two batteries at the alternator so when it is connected the second lower voltage battery only charges from the alternator and when disconnected the other diode to the primary battery conducts the alternator output if it needs charging. Ideally if you have room under the hood near the original battery to install a second one would be perfect and make it easy to move the power leads.
you can just add a switch and shut the alternator off. leave the output hooked up like normal. with the power not going to the voltage regulator it will just spin without trouble. I have done this to hook up an ac delco alternator to stuff and it is fine. The alternator is not hurt by spinning without power. When you turn it off it will probably take about 20 seconds for it to quit putting out power but that is not that big of a deal. There should be a wire that gets 12V when the key is in the on position and that is the wire that energizes it. Trace it back under the dash where it would be easy to hook a switch to it and you are done Just post where the wire is and what color it is so us lazy people can do it the easy way.
You can take the regulator out and use an old chrysler external regulator if you want and mess with it slightly to get the alternator to put out any voltage you want up to around 70volts. I have ran straight 12 volts to the brushes and used an alternator as a stick welder before. They work great as a cheap welder. The windings at full load at 12 volts on the brushes pulls around 4 amps through the windings depending on the size of the alternator so it can burn up if you weld with 100% duty cycle.
I will be adding a switch to my alternator soon to disable it eventually but it has not been much of a priority since it was really cold and messy outside.
Second battery - Simple you leave the alternator alone connected to the standard "starter" battery because most are pretty much sealed units that can't be modified - the regulator is built into it and connected physically to the internal diode assembly. Take one apart and you will see what I mean. But you run the rest of the car off the second battery that you allow to run down and then charge it when you want to. This results in virtually no alternator load connected to the original "starter" battery except for replacing starting motor energy which you need for EOC restarts. And you run off the second battery for all the other running loads until you need to recharge it. You always keep the starter battery topped off on charge to make it last and you can beat the second battery knowing that you always have reserve power in the original that you can switch back in when needed and is already there for starting. Who wants to get stuck with a dead battery in the winter?