Actually, this got me thinking. As my modifiers don't light up, I've included this in my MCU Buttons Zone:
Code:
Solo NoAction
Shift+Solo Reaper "65535" // No op
Option+Solo Reaper "65535"
Control+Solo Reaper "65535"
Alt+Solo Reaper "65535"
Now, whenever I use a modifier, the Solo button lights up (the one above the Record button, not the channel Solo button)
Handy to know when your modifier is latched