My Legacy has a "dummy" gauge as well, so I poked around in the service manual to see what the schematics say. In my car, the gauge gets its info from the ECU. I think that would make it pretty hard to make it accurately move.
You folks might want to get ahold of the factory service manual page that shows the "combination meter" (instrument panel) wiring diagram and see if that's the case for your cars as well.
-Ryan