I was thinking along similar lines - if the BCM is powered up, and has incoming current of the right voltage & frequency, how would it know (or care) that it's coming from an external source rather than the motor/generator? So it ought to do all the proper battery control steps. For shutoff, there is probably a control signal going to the M/G to tell it what rate to generate at. Capture that, and when it goes to zero, turn off external charge.
Though I wonder if you wouldn't also need a relay or switch, so that incoming external current doesn't also flow through the motor?