I thought that 'machine-sensed' ie 2wire alternators, with internal
regulators, cutoff the output at idle anyway, that is what why the
ignition light comes on, to show that the field windings are not being
self energised, but that the field current is coming from the battery.

Once the alternator is producing output volts open circuiting the ignition
light feed won't cutoff the output, but will stop it 'booting' up the next
time the engine starts.

The point where you need to O/C is internal to the alternator,between
where the field diode and the ignition light terminal join, and the field
windings themselves. This connection may be internal to the regulator
hybrid, in which case you may have to use a 2pole switch and interrupt
both incoming connections before they join!


TTYL


Paul