My project was more about proving the feasibility of wiring OPP boards to an existing machine without changing any wiring of the pinball machine at all. While theoretically I knew it was possible, the proof is really in actually accomplishing it. That being said, it wasn't the best idea I've ever had...but it did work. My MPU board was completely corroded. I considered going directly into the driver boards, but that wouldn't have accomplished what I wanted to prove. (That would also be a fun exercise, but I ended up giving the old boards away for free in hopes others could resurrect some of their machines).
Rewiring a playfield takes less time than you would imagine on one of these old, relatively simple games. (Especially since you can reduce a lot of the wiring if you distribute the boards under the playfield.) Just have to feel comfortable with relatively simple electronics. There will always be at least one brief moment that you will feel the machine will never flip again.