I think I made it not clear, what I'm trying to achieve.
What I'm trying to do is, if there is an instance running, accessing this one directly, without starting the clickonce url. I searching for a solution, where I don't have to write a little program (which has to be deployed as well, ...), which checks if the app is running, if yes hand over the params, if not starting the clickonce url.
The background update is not really an option, because this "connecting to app" screen is still there and consuming time, and it's a must, that every user is running at every time the most recent version of the app.