This is exactly what I was looking for! Thank you so much!
A quick tip in regards to non-steam windows games. I found that if the application path in launchbox leads to a .lnk file such as the launchers found in a lot of GOG games the steam overlay does not work properly. To fix this ensure that in launchbox your application path leads to the .exe file. This will ensure that it works properly.
Also, I would love for this to eventually work with the steam link (or bigpicture mode). Any chance that it could in the future? If I add bigbox/launchbox to steam as a non-steam game it will work on the steamlink/bigpicturemode but the steamlauncher can't launch games.
(That should say .lnk not .ink in the picture.... whoops)