So now that we've established you can indeed shut down charging via the IG wire on the ND IR alternator in question, here's a little more info to answer Dan's questions:
In my RV6A, I have the IG terminal attached to my alternator "field" switch and the sense wire attached to the B terminal with about a 4 inch lead, just like the Toyota schematic. The light terminal is not connected.
From the test card on my alternator, the turn on point where the alternator starts charging is 531 rotor rpm. My pulley ratio is 1.57. Current output at 1400 engine rpm/ 2200 rotor rpm is 50 amps.
I have not tested shutdown at the typical 4500 rpm cruise rpms I fly at but clearly there is more than enough current being produced to keep the field energized at the rpm I did the test and video at. My thought, and I could be wrong, is that the stator outputs cannot be connected to the field windings in this alternator as the schematic I posted depicts. I await someone posting info confirming or denying that conclusion.
My Haynes manual specifies a 7.5 amp fuse on the IG wire. My notes show I have the same 7.5A fuse on my aircraft installation.
With regards to what's inside the alternator and where the terminals are attached and component layout, I'll leave it up to someone else here to get one and cut it open. Same goes on measuring some temps on a Lycoming installation. My testing so far shows the highest temps occur at idle and after hot shutdown where there is little or no cooling air circulating.
You can buy genuine Denso alternators here:
https://www.densoproducts.com/Automotive-Alternators-c245.aspx
These are rebuilds using only OE Denso parts. I think the prices are quite reasonable.
Official Denso parts site here:
http://densoautoparts.com/why-denso.aspx
A few more thoughts: The auto OEMs today generally have far stricter standards for component validation than general aviation. The ND IR alternator has been around for over 30 years and no auto OEMs have used external regulators for at least 25 years to my knowledge. If they thought external regs were more reliable, they'd still be using them. You simply don't see many alternator OV failures on GENUINE ND units in cars and the auto manufacturers cannot accept failures on the huge scale they produce at. I'll trust Denso's design, engineering and validation on tens of millions of alternators long before I'll trust the likes of some small aftermarket firms producing hundreds per year, especially those which show a 25% failure rate in less than a couple hundred hours in some cases.
Continue the analysis on this topic but please stop the useless conjecture.
I've gone to some trouble to produce what info I can track down, pulled my plane out into a snow drift to start it up and shoot the video and pulled my cowling to verify the wiring connections. Time for someone else to step up and produce some useful information which will enlighten us further about the possibilities of being able to shut down one of these units with a shorted regulator if you think that's a likely scenario.
In the meantime, I'm going to keep flying my IR ND alternator and lead acid AGM batteries with the conviction that the IR is never going to have an OV event in my flying career, based on my long experience with these units. I'd be a lot more concerned about other things in the airplane giving me a bad day.