Thank you, internet, for almost always having an answer to a question. In this case, I had come across an old Apple Keyboard II while rummaging for something else. The AK2 is an ADB (Apple Desktop Bus) device, and so no longer compatible with contemporary Mac hardware. However, according to Scott the Robot, a $16 microcontroller, properly flashed and soldered, corrects that one inadequacy. Next step, [USB-ifying an Apple Keyboard II](http://www.scotttherobot.com/?p=755).