Easier still than that method is to route source vacuum to the center vent pod. Tap off the ACC vacuum source and route it directly to the center vent line just behind the ACC controls. This only works if the center vent pod holds vacuum. If it doesn't, you'll have to physically jam the flap like you said.

Removing the center vent blinds isn't fun. There's a risk of breaking the spring clips and it's tedious working with the screw that holds the manual flap to the thumbwheel.

Sixto
87 300D