Random old keyboard idea I once had: I once bolted a bit of code onto TMK where you could switch the keyboard into a "daisy chain" mode so instead of sending HID packages over USB, it would send them over the UART but prefixed with a number based n the hot key. So you'd press hotkey + 2, and it'd prefix packets with the number 2. The firmware also listened to the UART, and if it received a package it would look at the prefixed number, and if it was 0 it was send it over the USB connection, else decrement the number and send it over the UART. So you'd have a chain of controller boards and each board would be connected to a separate computer and the result was sort of like an infinite KVM using cheap microcontroller boards for each node.
I had the idea to also make a firmware/board that would take these prefixed HID packets and translate them to ADB, but I never got as far.