@pinballjim, I believe VirtualPinball and Future pinball both support "Digital plungers", which, unless I'm mistaken, function pretty much like USB mice, only just in the x direction. With that, they can detect the extend of the pull and the release speed of the plunger.
It'd be cool, but I'm wondering whether the effect is really worth 140$.
I used the mechanical switch idea, which is, as you said, basically just on off.
But, at least in Future pin and Visual pin, it's a little different than you stated.
Those games allow you to press and HOLD the button to essentially "pull back" the plunger. when you release the button, the plunger releases.
When the "button" is connected to a real pinball plunger, the effect is very similar, because as you pull back, the buttons is effectively "pressed" so the onscreen plunger appears to pull back as well, until you release.
It's not a perfect emulation, of course, but it's pretty dang good for using some scrap metal and a spare cherry switch!