I think I'm on the right path, though haven't made the destination yet. I've created an ICF lower wall, with an upper pony wall that mimics a taper top ICF block (the outside layer of foam being thinner) with no foam on the inside layer; I've got the inside wall planes lined up the way they should be, but despite the fact that the "Hang 1st Floor Platform inside Foundation Walls is checked" in my defaults, the slab will not extend to the inner surface of the upper pony wall.
I have tried deleting the foundation wall all together, and rebuilding walls/floors/ceilings, but to no avail. As you can see in the photo, the slab still will not extend over. I am afraid there is something simple I am missing.