I found this recently:
It’s a USB trackpoint. Just the little nubbin on a PCB that’s then adapted to use USB to connect to a computer. Paired with this shim brewed up by another community member that exposes a USB 2.0 port by nixing the fingerprint sensor:
I think it’s possible to actually accomplish a Trackpointed Framework. It’d still require modifying the Input Cover itself (namely… the aluminum and the Keyboard are a little in the way so a hole is in order) but it seems accomplish-able.
Those USB trackpoints are also all over Aliexpress for ~$15 USD but I found it on Amazon first 