http://www.lynxae.co.uk/Products-alternators.htm
Dunno if you have seen these guys before, but their 140a alternator weighs in at 6kg, and they make their stuff with denso parts for most things.
As for "switching off" an alternator, i would only attempt it on a 3 wire alternator with a l-ion battery, as the voltage drop would be catastrophic otherwise.
Depending on ecu choice, you could switch to a 4 wire off a later model car to duty cycle control the alternator, so it doesn't load up when you don't need it to, and i'm assuming this is what Brendan is referring to, as thats how a lot of modern cars are. For example with a pnp m800 for sti's and evos the computer will switch off the aircon compressor and run the alternator at minimum duty to maintain voltage at high throttle inputs.
Ultimately your 100a requirement will be the killer, you can find some 3kg 80a 4 wire alternators fairly easily. Maybe you should adapt a suzuki swift or similar steering coloumn into your car and run a manual rack.....