I don't have any triple digit models, happy with what I have, they all still work

but went and disassembled a few thinkpads to refresh my memory, all the ones here, the buttons below the touchpad are controlled by the chip on the PCB under the touchpad. The buttons (2 or 3 depending on model) above the touch pad are part of the keyboard which would be logical, keep them as part of the stick so use a different controller though really, everything is programmable.
However..
The two buttons below the touchpad on my TP R61 are not integrated, they have separate lines via a lead to the touchpad controller which are simply directed to the plug that goes to the MB.
The two buttons below the touchpad on my TP R61i (model without fingerprint thingy) go through the touchpad controller, they do not have separate lines so it seems to be very much a try it and see thing even depending on similar models. Not that this helps Andi but it does answer a sidetracked curiosity.
Maybe Andy's pointing to his setup will help Andi, if not looks likes it's time for some foil. I would not cut any Mylar cables unless you have spares!