Your welcome bud.
I've got the button mod sorted for the Seimitsu PS-14-KN/GN's if you are interested but it should be able to applied to most other buttons too. I am using buttons with a translucent plunger + inset artwork so that's the first button I worked on.
As you know there is much less button to play with than the Happs which makes it much harder to remove material from the plunger, mainly because its so damn hard to hold the thing!
Anyway I eventually came up with a fixture to hold them and removed 2mm from the face of the plunger:
I'm then going to turn some 2mm discs of cast acrylic to light in the same way as your button mod. I made some nylon discs for a start just to check the idea worked. I'm going to put a piece of aluminium foil in between the plunger and the artwork to stop the LED lighting the artwork:
You can 'just' fit two 5mm holes in the bottom of the buttons for LEDs.
The snap in type buttons have a smaller microswitch which can give you a little more room to work with.
However mounting the LEDs here means get a lot of spotting as the LEDs are mounted directly below the edge of the disc, unlike the Happs mod where the LED is in the center of the button. To counter this I am going to make some black nylon LED holder inserts to mount the LEDs as close to the microswitches inner shaft and center of the button as possible.
If you can't do some of this yourself maybe I could trade you for the work you are doing for me?