I know you mentioned factory but I'll offer that I've done Rostra electronic cruise installations on 2 1150RTs (similiar to the S) with good results.
I touch on it at the bottom of the string (Craig G) here:
https://www.bmwsporttouring.com/topic/89062-r1150rt-cruise-control/
I thought I was being original by connecting the Rostra actuator cable directly to the throttle tube as I had not heard or read of it being done before. This eliminates the need to mess with the Bowden box thus simplifying things dramatically. Additionally I used the factory switch as your pic shows. As I mentioned in the RT forum this setup has worked flawlessly for 25K miles...
I pieced together everything I need to do the installation on my S but I'm not sure I'll ever get around to it.