Sorry for the necrobump - wanted to add for relevancy and it doesn't make sense to start a new thread. I just had the same problem with Anodyne running as a Steam game and resolved it.
As Jason mentioned, there's probably no way to reliably read an exit code for a game running from a Steam ID. You can, however, edit the game profile in Launchbox to use the actual executable under <Program Files (x86)>\Steam\steamapps\common\<Your Game>\<Game.exe> -- "Run After" applications should then work as expected.
Not sure if this will work with all games, but it works with Anodyne at least.