Well, since from the discussion above it seems my way is as easy and efficient as any other, and not remotely complicated, I think I'll stick with it, bar some day finding a way to force my keyboard to type "smart quotes" as I go.
i.e. a) edit in Sigil correcting quotes - using keyboard straight quotes as needed, then b) converting ALL quotes to straight via search/replace, c) using Book Polish with smart quotes checked - alll quotes are fixed to curly quotes. Three simple steps and it's done, so I suppose there was no point whining in the first place.
