When I measured the VSS signal (TAA pin 2 on the K12S GPS connector) with my CRO I got a noisy sinusoid, not a square wave. Beats me why BMW would offer up the plug and the pin but make it near useless...I can try to post an image later perhaps as I took a photo. I asked for a software update to make it functional but BMW said it couldn't be done...go figure!
The VSS pin 2 signal comes from the ZFE module. At idle it still reads around 6mV (same noisy sinusoid) and it does change with road speed in amplitude, not sure if in frequency (CRO is a bit large to have sitting on the tank!). So perhaps you are right, it's a coil based signal. I think for it to work the frequency would have to increase with roadspeed, not amplitude.
I wouldn't mind using a working VSS also for the PC-V I have fitted, but I digress.
I'm against adding additional hardware if I can help it, just the potential for more failure modes and ugliness on what is otherwise a beautiful bike. YMMV.
Perhaps the GS and K12S have different GPS connector functionality. Would be great if you could read/capture with a digital portable scope what the GS puts out on pin 2 wrt ground.
FYI, there's also a wire called "WL" between the instrument cluster and the ZFE - anyone know what this wire is/does? Refer https://files.me.com/kpavey/f4skft