Just had a peek in the wiring diagrams, and can't come up with anything I'm afraid.
Only thing I can suggest is to look for a small pipe that runs from the turbo charger unit, and see where it runs to. If it runs to the ECU, then there must be an electrical output to send to the dashboard.
As yours didn't originally have an electronic dashboard, try tracing the boost gauge pipe from the dashboard back into the engine bay and see where it leads to. Might give you a clue as to whether anything is measuring boost (ECU?), or if it goes straight to the turbo charger unit.:idea: