You are correct in that it is a vent hole. I have seen ones with the electric choke, but usually the little shut off solenoids are in the fual inlet passage. With the solenoid in the bowl area what will happen is if it is shut off and the motor is still running then the bowl will overflow. There are two vents on the K series carbs. one is located in the carb throat on the bottom of the opening< this one is the main jet vent, if this one gets plugged then it will surge rich on top end. The other is the one discussed here. If it is plugged then the fuel bowl will overflow (an atmospheric pressure thing). I agree that you might pull out the solenoid, cap it with a welch plug that you can get at any good hardware store for about $0.10, then drill a vent hole on the flywheel side of that boss. Hole size from 0.062 to 0.0937 should be plenty. I agree that if you are having problems with the choke then pull it out and replace it with a manual set up. These can be found lots of places.
In looking at your first picture I can see there is already a vent on the front of the carb opening about 2 o'clock. Again, you can still drill an external vent hole and will be fine. All our race carbs have an external vent hole.