I was wondering if you were trying to rig something more sensitive, where the spring extension length was somehow converted to force used for launching the ball. Trying to think how you would do that electronically.

Not much point in it, since Visual Pinball just uses the amount of time you hold down the plunger key (Enter by default) to determine how far to pull the plunger back. The software itself has no mechanism for any kind of analog input or anything like that. So, for this application, a well-placed microswitch is the best fit. Not as realistic as what you're proposing, but I don't see them adding this kind of feature in Visual Pinball any time soon.