It's not a desktop PCIe slot on the back of a laptop, but the interface is already documented. The electrical, shell-fit PCB, and general form-factor specs of the 16" laptop's expansion bay are already published under a CC-BY license by Framework: https://github.com/FrameworkComputer/ExpansionBay

Direct link to the pinout PDF: https://github.com/FrameworkComputer/ExpansionBay/blob/main/...

It's a 4th-gen x8 interface, so there are limits to what "any old GPU" might be capable of performance-wise, but creating a passthrough-port accessory seems possible.