Temp gauges have been useless on BMWs for more than 10 years. The E46's gauge was run by the computer (not directly from a sensor as of old) and displayed essentially three things: cold, just right, and hot.
You can get an OBD reader that displays temp. Can't remember the name right now; search.
You might be able to get someone to code the dash display to display one of the engine temps. Pretty sure that even the thermostat in our D's are computer controlled - they have been on the gas side for over 10 years. If you follow TDI's posts, the engine never warms up anyway, so why bother.