I think that suppressor is probably something intended to protect fragile gasoline fuel injection system electronic control components, not needed on the diesel engine. If it's labeled "W" then I would suspect it is a real W terminal so you are in luck there! You can probably discard the suppressor and just connect your tach to that freed up W terminal.
I wonder if a Jetta TD regular diesel tach would swap into the Quantum dash and be fed by that W terminal. Though as you mentioned the ratio of pulley sizes might not be quite right and through the reading off so you would need to adjust the frequency with a signal converter anyway. At that point, maybe just using one of those Dakota boxes to do it all and hook right up to the stock gas tach might be easiest.
Looking great, will be fun to see the first maiden voyage on the road.
