Not sure if you can share the details at this stage, but if you do, how keyboard connection is implemented in hardware and software for Android? TOHKBD has a daemon which connects to userspace-exposed I2C and GPIO interfaces, reads the keys and in turn creates userspace input device. If it's something similar, the daemon code could be mostly reused.

I would like to try porting SailfishOS, but Moto Z is far from the budget device, unfortunately.

Edit: Found https://developer.motorola.com/explore/firmware. The SailfishOS software will probably need to talk to "Greybus" interface then, unless keyboard is exposed as proper HID device by kernel.