^^^ Huh...yeah, a servo would have been a lot easier. But the challenge is what drew me to the project. I'm in it to learn.
If I connect it under the playfield, I'll end up using 4 separate boards: 3 to intercept the lamp matrix wires on the three playfield tach lamp boards (even then, there are a couple lamps that will need to be desoldered and rerouted to a board), and another board for the micro and tach driver circuitry. Or maybe I can run my own little lamp matrix in parallel with the real lamp matrix and tap into it right off the board in the backbox. The latter would allow me to keep everything on one board. ...but I haven't thought much about how/if a parallel lamp matrix would work. It's definitely going to be a lot of work to integrate.
If I don't do anything to detect attract mode, the needle will probably peg because, if I remember right, all the tach lights flash during attract. I might be able to differentiate attract mode from gameplay by watching the lowest tach insert lamp. I think it flashes during attract, but is solid during gameplay.