How would a a stock DSC respond to the M-Drive request ?
I don't think it would respond at all. The message handling might be in the non-M DCT logic, but the sport button config pays attention to the sport button and little else IIRC. The Z4 uses CAN for multiple selection options instead and I think only uses 0x315, which is separate from MDrive/0x399.
You can obviously change the defined settings that are used on sport button or Z4 modes in the DCT bin, but the only shot at non-M DCT using MDrive is probably to configure without sport button logic, make sure 0x399 logic is enabled if it's there and then there's no guarantee a non-M GWS would even pass it.
Remember when I accidentally left the M3 GWS default coded as 1M and couldn't figure out why 0x399 wasn't making it to the trans? It worked as soon as default coded back to M3. I'm not sure if a non-M GWS could even be coded to do it, even if everything else was in place. Maybe F8x coding?
Maybe
@Olza can shed more light, but if you were going to go through all of that, I'd either leave what you have or just buy the MGWS and make it easy and sure to work.