Well, I'll start at the beginning of the list...

what are you using for the front plunger? Does that actually activate a switch?
That's an actual pinball plunger shaft I had on hand. It has an "E" clip on the front, a pinball plunger spring in front, and a washer against the cab wall. On the outside is another washer, and a spring, then the pull handle. The spring is too weak though. Right diameter, wrong thickness, so it crushes easily. I used part of a pinball coil sleeve so the plunger wouldn't just wear on the bare wood hole in the cab. I plan on running all switches through an IPac controller board which should give me every keystroke combo I could ever need.
Is there a way to make a plunger that works really well with pinmame?
Since the "Enter" key is what is usually used to launch a button in pinmame, I'd just do what someone else here did, and use a leaf switch. The longer the button is held, the longer the plunger is pulled back. On games with a fire button style (Indy Jones, Shadow, etc.,), pulling the plunger would auto fire the ball, just as a regular fire button would.
Are you going to have a lockdown bar and apron too? I'll be interested in how you'll do the nudges, as I haven't added nudge controls to mine yet.
I plan on it. Keep in mind that this particular cab is just a preliminary test bed. Once I work out any "kinks" in the design, I plan on making a full size pinball cabinet.

I haven't completely worked out the nudge issue, which is really the whole reason I built this. I want to avoid any additional external buttons that you wouldn't have on a regular pinball machine. Things like an exit game button will be on the bottom of the cab where you normally can't see them.

I've been given the idea of using the lockdown bar itself, combined with tension springs and switches to do the nudging, and the concept is really growing on me. It seems do-able.
Essentially the bob would sit directly in the middle of these 4 sensors... EXTREMELY close to them, so a bump to the machine would cause it to hit the appropriate sensor depending on which direction you whacked the box. The only issue I see with this is, you whack it hard enough that the bob bounces around and hits multiple sensors as it swings back and forth. That would be a bit innacurate I guess.
Good idea! The accidental bounce issue is what is keeping me from jumping on the mercury switch idea right now. I don't want extra "bounces" if possible.
Good questions and ideas everyone!
Russ