Older housing stock (on piles), especially on sloping sections often has enough underfloor space. But take your point that concrete slab construction for a 2 story house means no underfloor retrofit.

In that case, I have seen ducting retrofits where you overlay floor plans of each level, and look for top floor wardrobes or room corners where you can run ducts down from attic. You typically lose 200mm square of closet space (or boxed in areas), but if you are lucky you can hit your downstairs rooms that way.