Last night I got a whole piece further. For analog plunger functionality (Shooter Spring) I tinkered a construction with the 10k potentiometer kept parallel to the plunger and the slider of the potentiometer is pulled along by the plunger. With two plastic discs which fits the slide, it works perfectly. One screw from the plunger attachment was replaced by a threaded rod. On the rod I've glued the potentiometer with two-component epoxy - it was an idea I copied from Sven from konsolenhal.de.
Now in the games the spring can be positioned accurately for skill shots. If you push the plunger, an "Enter" is triggered, which is the ball-firering-button in some tables such as "Iron Man". Therefore, I could save that extra button. The potentiometer is also connected to the controller PinAna 1 that offers analog nudging with his acceleration sensor. I had already written about Pinana 1 and the big brother Pincontrol 1.
Here are a few photos of the construction and at the bottom you'll find a little demo video where you can see the function: