Altho the following refers to thumb pistons, the same principles apply to using buttons for stop control. You might get some ideas here:
http://forum.hauptwerk.com/viewtopic.php?f=15&t=13905&p=102980&hilit=thumb+piston+LED#p102980or here:
http://forum.hauptwerk.com/viewtopic.php?f=15&t=12264&hilit=+thumb+pistonsLikewise, if you need to drill your own board(s), suggest using a proper diameter Forstner bit (with a back up board) as this combination makes for a clean hole. This setup along with a small drill press and a simple fixture works well.
One thing to consider is how deep is the push button/LED assembly. By depth meaning how far does it protrude beyond the front of the board so that there is sufficient space as to not short out or touch anything such as the keyboard mechanism and likewise enough room for the wiring. It might be possible to add spacers if needed.
Roman's LITSW boards with the proper Master Controller is a good choice. You can program the button operation to be either dependent or independent.
Rgds,
Ed