Often1 gave me another idea: if you don't mind turboprops, fly to BHB (Bar Harbor, ME) from BOS. Both 9K (Cape Air) and 3M (Silver Airways) fly BOS-BHB; it would probably have to be a separate ticket from SFO-BOS, so the usual separate ticket risk would apply. But it's hard to beat flying into BHB to go to Acadia.