Apparently you can buy special 'electrical' paint on the
internet to re-paint the buttons as it is this black material
on the underside of the buttons that completes the circuit
on the circuit board and registers as a 'press'. At $25
a bottle however, I cheaped out and bought a small bottle
of 'rear window defroster' repair paint. This is used
to repair the 'metal' lines on your rear window defroster
to complete the circuit and works pretty much the same
way on the buttons. I got mine for about $8 at an automotive
store.

The only negative I've found is that the paint is really
not meant to be 'pliable' when the button moves, so over
time it can sometimes flake away and you have to re-apply
it. You also have to really clean these flakes off or
else they will affect the operation of the other keys.
(Randomly completing circuits!)

The only other option that I found on the net was some
guy Krazy glued small pieces of tin foil to the underside
of the buttons. The tin foil was the same idea as the
electrical paint so pressing the key then cause the tin
foil to touch the circuit board and complete the circuit
resulting a button press. Same theory, just different
material.

Of course if Harmony would just sell you a replacement
keypad at a reasonable price, that would be easier. I've
never thought to ask so that's another option.