I have successfully used black electrical tape on the back side of the lens for the purpose you describe. I discovered that, after the lighted buttons were installed, the magic marker mod wasn't enough to completely hide the innards. So I tried the electrical tape thing.

It required some trial-and error with the cutting to get it right, but now I've got no light leakage, in or out, where I don't want it. Since I don't use IRDA, even that's covered up, I only have a hole for the IR remote sensor.

The beauty of using the electrical tape is that it can be peeled off without damaging the faceplate.