I understand that the Ducati regulator for the 912 ULS is not a linear regulator (not series nor shunt). It connects the generator windings directly to the pins +B and R (after rectification of course) when the voltage on C is 3-14 and opens the circuit otherwise. So it essentially sends pulses of full voltage and the battery or capacitor smooth it so instead of pulses of 0 V and up to 40 V it outputs a mostly steady 14 V.
If the C pin is disconnected then the RR will go into an overvoltage condition and keep the generator tied to the bus (peak voltage close to open circuit stator voltage). What is the voltage drop in this case? It would be the sum of the rectifier's diode drops, any other internal diodes, and the transistors in their on-state. So for example if the generator is producing 40 V RMS and load on the bus is minimal what is the DC voltage on +B/R if C has 0 V?
