Where you're headed with this is what I wanted to create for my GM stuff, I'm just not that good at writing code, other than assembly, I seem to have my head pretty wrapped around that...

I was hoping that the TVII that I have in my car would have done this, it was supposed to, but since the creator of it has backed off from pretty much all GM support it makes it almost useless. There's only a few things that can be displayed on it, that can't be done with regular gauges. I'm using mine right now mostly as my temp gauge since I don't currently have one in my dash.

Anyway, the way to go with this, hardware wise is SMD devices to make it small and fit INSIDE a gauge, with only a few external connections. But like some gauge companies are doing you could make one control box that feeds one or more gauges on a common databus, SPI, I2C, etc.

It's not hard to follow the pinouts and figure out what connects to where, there's enough information on the interwebz to help there, especially since most shields schematics are available, you can pick out what you need and leave the rest alone, and connect the appropriate pins. Take a look at the Teensy, it's a pretty atout bit of kit.