On later model cars (like the N63 engined cars), BMW certainly does program the DME with a charging strategy:
Enginerdy: Why BMW's N63 twin-turbo V8 eats batteries
However, firstly, we're talking about a voltage SPIKE, and no matter how fast the computers are, they will still see the spike, before it can be controlled.
On Thursday, I also had an E60 M5, that was intermittently lighting up the dash with almost every warning light, as well as registering a list of warnings on the i-drive screen. When hooked up to ISIS/ICOM, there were literally dozens of codes, in a dozen computers, many indicating bus line communications failures, steering angle sensor failure, wheel electronics failures, speed sensor failures, and SRS issues.
But the DSC, the SRS, and a couple of other computers showed codes for "Overvoltage > 16volts". The DSC registered this code 34 times.
The alternator voltage showed exactly correct on a DMM, and also through ISTA, so I cleared the codes, and drove the car to see what came back first. In a trip around the block, the overvoltage codes reset.
Therefore, I ordered a new Valeo alternator, replaced it (3 hours!),cleared codes, and drove a long test drive......no codes, no lights.
My point being that while the DME has maps/strategies, at least in later cars, for controlling alternator output, it can't stop that overvoltage until AFTER it occurs, by which time the overvoltage can have already fried a computer or two -- and often does. It's worth noting that there are different codes for "Overvoltage > 16volts" and "Overvoltage > 18 volts". There would be no such codes written into the programming, if the DME could indeed prevent the overvoltage from happening.
The E60 M5 was lucky, no further damage.