In my idea the cams would sit between the nacelle and the spinner. It’s a two-part assembly. Fixed to the spinner, between the spinner and the nacelle, you would have concentric tubes (or arms) with holes at the bottom of each for the tethers to pass through. You would have a minimum of 2 of these concentric tubes if you use a single cam. Jutting out from the nacelle you would have the tubular cams. That is a part that can move in and out, moving in and out between the concentric tubes, depending on if you want to do cycling banking or not. It can also rotate if you need to compensate for different tensile shaft twists.
At first you could try not actuating the cams and just see if a fixed position of them works well enough.
The height of the tubes would be matched to the height difference of the cams. And say you use a single cam with a 30 centimeter height difference, you could vary the line length by a maximum of double that. You would need to extend the shaft coming from the nacelle then by 30 centimeters if you don’t allow the cam to move in and out, and by 60 centimeters if you do - unless you mount the two parts around the nacelle and spinner. So it might be better to use more cams instead, if that doesn’t make friction much worse