Taking them apart and using only some of the keys is very very difficult one of the reasons being that thee is nothing to solder wires to.
Taking them apart and using the controller board to generate just a few keys worth of data, using "other" switches for the actual contacts, is pretty trivial. Here's a keyboard controller that is connected to a dual-footswitch that generates "forward arrow" and "backward arrow" keystrokes. No arduino required, even. Works with keyboards of any technology (PS2, USB, ADB...) since the interface to the computer remains the same...
