Sorry buddy. Your first question was clear, but all the reactions on it, made your question looks like you asked something different.
It is not much, but this is the information I have :
So, you can only use direct EXE files as table name, with no parameter's at all.
And the exe must be exact the same what you launch, other wise it cannot monitor the process and goes back to PinballX.
So no go for Steams app that needs steam.exe to launch (maybe a feature request for adding Steam games would be a nice to have ?)
I would update the PinballX documentation with the following note :
System Type
Specify a system type. If other than Custom, PinballX will treat it as the specified in built system and allow features such as high score support.
Note: ‘Custom – Exe Files’ doesn’t require any of the next parameters. This type just points in Game Manager to the full path of an EXE file and monitors this process until it stops. Pressing the exit button will exit this process as well. No Parameters can be used