So I read through the Microsoft HIC over I2C spec and one unusual thing is that the device’s I2C address has to be specified in the BIOS ACPI info. Everything after that point is plug and play, though. So it’s possible that we’d need an I2C device address translator to ensure the ThinkPad touchpad uses the same I2C address as the stock framework touchpad (7-bit address 0x33). Those exist, though.
(For future reference, USB-to-HID-over-I2C bridges also exist, which would let you use the built-in touchpad as an external USB device, if anyone wanted to do that.)