Update: I have this working and have been using it for some time now. Just yesterday based on a discussion with @Arnd earlier this week that I took as a challenge, I added a router spindle relay to the arduino. Since I had the controller apart, I also fixed a broken ground wire for the status LED so now my raspberry pi responds to button presses and displays a tricolor changing LED that is configurable in webcontrol to indicate its status.
Picture:
before cleanup and reinstall:
In operation. LED above display is blue in standby and green flashing when sled is running.
the LED is adjusted or changed in webcontrol->settings->GPIO settings
Because the display takes up most of the GPIO on the rpi, only pins 5, 6, 13, 19, 26, 12, 16, 20, 21 are exposed for buttons and for LED. Currently, there is one pin remaining that could be used with the way the controller is wired.
If anyone wants to try out putting buttons in their controller, it is possible with a little effort. I also have access to the laser to make the acrylic faceplates, and I can get you a custom one at a reasonable cost (message me).