The ferrite beads act as inductors and help to cancel out any unwanted ripples or spikes. Were you in the US I'd mail you a couple, but you can sometimes find them pretty cheap. Any PS/2 cable should work, but you might want to try a beige one to look nicer.
What I would do is drop a teensy in there, and put soarer's code on it, then run a USB cable out. Up to you of course.
Ferrite beads (I believe) can go anywhere on the cable, but it's usually more useful to have them near on end.
One issue I have run into with a cable swap (that you may or may not) is ending up with a hole in the keyboard where the cable comes out. You may want to look into a cable gland to provide some strain relief and help fill up the hole a little.