The source for disconnecting the regulator from the battery being externally charged comes from a US distributor for OEM type DucatiEnergia regulators (EME/EuroMotoElectrics):
FWIW, I was always concerned about pulling the 30 amp charging fuse out, and replacing it, since the connections are already tiny and under-rated for the amperage. Same for pulling relays out: too easy to spread the female contacts below and disturb the connections. I feel like I solved this with by replacing the 30 amp Charging/Regulator fuse with a rated circuit breaker. So, yes, easy for me to "disconnect Voltage Regulator/Rectifier from battery" while charging externally. Makes a difference? One can hope . . .
One thing for sure, EME has been proven totally correct that "Additional wire ground(s) direct to battery and/or engine are strongly recommended."