Working off of this project for now:
https://github.com/dawsontoth/zwifit I had already been monkeying around with the Pi and Ã…NT+/BTLE when looking for a way to multiplex the signal from my Kickr '14 but then I upgraded to a Kickr '20 and didn't need it anymore. Was using a footpod on the treadmill but it's "accurate" to the speed you calibrate it to and drifts above and below. Not a huge deal but was always mentally annoying. Was planning on some buttons/display on the Pi to manually set speed, but then found the above. It's a little flaky but works most of the time.
I set the treadmill to a designated IP in the home wifi to make this easier but Zwifit will scan the whole subnet I think.
I am using it just as a relay from treadmill -> Pi -> Zwift right now, but the websocket will accept commands as well. Zwifit has a basic UI that you can use to change treadmill speed/incline.