I can understand how turning on the headlight lowers the system voltage and the RR offsets this by increasing the alternator output to bring the voltage back up but doing that cannot change a half wave rectifier system into a full wave system........
I understand your frustration. Looking at the schematic it LOOKS LIKE the RE uses a permanent magnet spinning around a three phase coil to produce electrical power.
Most japanese bikes use a ferro-reluctor that spins within the magnetic field created by the "field" coil (the field coil is controlled for intensity by the regulator) and then the 3-phase power is extracted, and, rectified to feed the electrical system.
the RE needs some sort of passive regulation system that either:
1. burns off extra energy to avoid over-charging
-or-
2. adjusts the resistance between the bridge rectifier and one of its output lines.
I suspect the latter.
oh, and GHG's suggestion of thyristors would fit that requirement nicely!