These Diesels don't generate intake manifold vacuum like gassers do so there's no direct way to use vacuum as an indicator of load. Transmission vacuum is an approximation of load based on throttle position. If your car has a transmission vacuum amplifier (blue flying saucer thingy), the vacuum signal to the transmission is integrated with boost so it might give a better interpretation of load. I suspect what you propose is a Rube Goldberg interpretation of a throttle position sensor more than a load indicator but in general it should provide visual guidance as to how your driving style affects fuel economy.

It's a fuel flow meter only to the extent that throttle position correlates with fuel flow. Having a transmission vacuum amplifier gets you closer than a simple throttle position indicator.

Sixto
87 300D