Use a small leak in the tube feeding the diaphrams after the non return valve.
There is a bloke in my town who recons diaphrams who is only too happy to make boost retard set ups for people.
Just beacause you disagree with a given way of doing things, doesn't mean it wont be effective. Everyone would love to have a fuel injection manifold for to use on a turbo A series. But they don't grow on trees. You make do with what you got.