I made some more progress this week. The factory petrol tachometer has now been converted to work with my diesel van. The red line is different, but the revs seem to match my aftermarket tach very closely.
Also, I felt bad about taking apart my Jetta tachometer, so I decided to try out these tachometer PCB's I had made. Turns out they work! I've also figured out how to bench test them with a signal generator now too, which makes calibrating them much easier.
I still have some more ideas/changes I would like to make, but I'm pretty happy with the progress.