Whelp - I finally had the stars align and I had the needed parts, and the time, so I got the new Denso alternator fit to the S2000. The alternator itself is a good bit lighter and that was expected (the Honda unit weighs 5.2kg) so that was easy weigh loss. I also ditched the stock belt tensioning system and that saved nearly as much weight as the alternator did. In total the weight loss was 3.9 kg (8.6 lbs) and as an added bonus it comes off the driver's side of my car. This should bring total car weight down to 545 kg at this point.
The alt charges at the desired 14.6 volts and I spun the engine up to 9000 rpms a few times and nothing went bang and came fly up through the engine cover so I think I'm off to a good start. Once we get some warmer weather I'll put some miles on it just to be sure but I suspect it will work out fine.
Maybe now it will finally start snowing here so I can stop fussing around in the garage and snowboard instead...a boy can dream during the warmest and driest winter in 30+ years here.
dave