A distributor has a circuit which has a vaccuum advance; a hose leads from the intake manifold to the distributor.
Can this same circuit…as is…also provide timing retard under boost, or must some internal modification take place?
I would say no. A typical vacuum advance unit is spring-loaded, with an internal stop to keep the spring from pushing the mechanism beyond the rest point. That stop would prevent pressure from moving the diaphragm. All of the vacuum retard units I’ve seen (admittedly on non-turbo engines) use a separate vacuum unit mounted the other way round.