Thanks for your reply. Unfortunately, this does not work, because the LaunchBox window is only temporarily activated, and the Steam window is re-activated again after a second or two. This is because when the BigBox.exe process terminates, Steam assumes that the "game" has closed and returns the user to the Steam launcher. Even without this issue, having to use Ctrl+B or the LaunchBox menu within SteamLink is rather painful.
The best solution I have come up with so far is to create a KillLaunchBox.bat batch file, add that as a Non-Steam Game, and then launch it from Steam when I need to kill LaunchBox in order to run BigBox. Of course, this is pretty clunky and runs the risk of issues if LaunchBox happens to get killed in the middle of updating its database or other persistent state.
If BigBox could gracefully terminate LaunchBox when it is started, that would be an almost ideal solution.
KillLaunchBox.bat