Rewiring all the keys seems a bit overkill to me. ADB, just like USB, uses 5V, is fairly well-documented (https://developer.apple.com/legacy/library/technotes/hw/hw_0...), and "A common implementation platform for the protocol is a simple microcontroller: a 2-MHz Motorola 68HC11 has been used with success, although any number of microcontrollers would do a more than adequate job."

Wouldn't it be easier to write a ADB-to-USB converter for the particular hardware, and put that between the USB cable and the keyboard?