I found a cheap USB keyboard controller chip! It is the Holtek HT82K629A. It requires a minimum of passives, plus a 6MHz crystal, and you're done -- just hook it up to USB and it looks like a standard keyboard.
You don't get to say which key is which: a given intersection of row and column is hardwired to be a given key. I've hooked up the "1" key. When I plug it in to my computer, it works great. I can type 1's all day. So now I don't need an FRDM board to run the keyboard.