Blow through with vacuum secondary Holley Carb.

Seems to me on boost there wouldn't be any vacuum to open the secondaries.

Except the 'vacuum' is just a pressure drop as the air accelerates through the venturi. So long as the whole carb is pressurized, it should be fine. It's the backside of the actuator that needs to see the same boost as the carb throats.

Doesn't mean it will work 'fine', but it should work.