My premise may be way off, but isn't the open lead output voltage linear
with RPM?  So my thinking goes, if you get charging voltage at idle of
1000, it will be 6 times higher (over 70V) at a cruise RPM of 6000.
The Yellow Daisy it is! You are correct in your assumption that the POTENTIAL output increases with RPM but it isn't really linear. Something to do with internal coil hysteresis or histowhatsit. The coils/magnets deal with both sides of the sine wave in modern alternators. I won't get myself any deeper, I'm a MECHANICAL engineer, don't have the full sparky thing in the memory bank.
Bill