Well this turned out to be MUCH easier (and more obvious!) than I would have expected given that it's remained a mystery this long. I found someone who offers paid coding and lists AHL2 activation among his options. I emailed him explaining that I was already a fairly experienced coder but couldn't find out how to enable this one feature or even whether it was possible for my car, and told him that if he offers remote coding I would pay him for this single feature. He generously replied back just telling me how to enable it!
There are no VO changes required. The setting is in the NFRM module, and its super-secret name to disguise it from everyone is...."AHL2_ENABLE". You just have to set that to "aktiv". My M3 now behaves exactly like the video in this thread.
I'm thinking this setting gets defaulted to enabled or disabled appropriately by region because the car's model code in the VO is different based on region; for example, my M3's model code is KG93, which I believe means "US-spec left-hand drive M3 Coupe".