I have. Not for pinball, but I have. It works fairly decent. Really depends on the printer you're using and how it puts down ink. One thing you might look into is with the artwork you're printing, when you're printing the black, make it a full process black. That means, if you're doing the artwork in Illustrator, make all four color sliders 100% of each of the four colors: Cyan, Magenta, Yellow, and Key(black).

Some laser printers will print this kind of "full" black with all four colors and make it MUCH darker with just a single pass.

If two stacked decals is all it takes to achieve the required opacity you're looking for, you're pretty lucky! I'd just do that for all the inserts, then, and not worry about anything else. Just make sure you're covering the whole insert with decal material. That way, when you go to clear over it, you'll have less sanding time as it'll be more even to start with.

BTW, I can't wait to see how this game turns out!!