K I've done my research and here is my conclusion:
It's impossible (for me) lol
You are absolutely right on the brake sensor stuff. MSA system do require the brake vacuum sensor to be in place to function or it will throw a code and fail. Also this code can get DME into limp mode for some reason.
About JBE, I was not able to rule out if it is necessary to code but if I do, it will start to probe for the dc-dc converter that is almost impossible to retrofit (a LOT of wirings), and the code for that also stops MSA from functioning.
Now finally DME... yeah it needs to be coded somehow to activate start/stop. Since a 335i N55 with MSA simply never existed I can't just swap the right one in. MHD offers some options like popping, cold start cancelling, etc. I know those are OEM options that are acually codes lying in the DME just like any other modules, I suspect MSA is on the same level. However, I have no idea how to reach it.
So in order to do this retrofit, I'll need:
new brake booster (so I have the hole for the sensor)
brake vacuum sensor
dc-dc converter (and whatever the hell of a wiring job)
DME coding (no idea how to do)
I actually reached out to BPM Sports who actually managed to retrofit MSA into an M3. But they said it is not worth the time effort for them to look into it. I think I now understand what they mean.
I quit
