I have a 2bbl Weber progressive that doesn't have a ported vacuum.
I want to add one to use with my vacuum advance.
It looks like a 32/36 DFV, and there is a plugged hole just above the idle adjust screw.
That's where the 32/36 DFV has a vacuum pipe.
On this inside, there are four tiny holes just above the throttle plate.

I am not intimately familiar with that Weber carb, but the port you describe, leading to very small holes above the throttle plate (when the plate is closed at idle position) sounds like exactly what you're after for a distributor vacuum signal.