I was unable to do the same thing with the buttons due to their complex shapes.

But aren the holes in the backs of the buttons where the switch and LED go in round? You'd have to put your piece of milky plastic at the end of this *round* hole. OK, it's going to be a real PAIN if you ever want to get it out again, but it's worth a try isn't it?

I can't imagine milky coloured buttons looking as good as the current ones, but I guess that is something to try too.

How about changing the plastic in the back of the button to make it concave so that it works a little like a lens to scatter the light? It might not work as well on the top lens that is very convex on the outside, but it may work on the others ...

Some automotive switches, radios and the like also have quite complex prism/light pipe arrangements to spread the light from one source to several different places - I can imagine something like this being used to light up the knob evenly. Would be rather complicated to try and make something like this though!!!