Back upstairs, above the work, we pounded in some shims to fill the small gaps between the new 2x4 bracing and the subfloor. Much more rigid now!
Next step was to cut some plywood to fit over the 2x4s and fill the hole. We cut a couple of pieces of 1/2" plywood. The first piece was glued and screwed to the 2x4s, the second (same size) piece was glued and screwed on top of the first piece. Success!
Final task of the evening: cut plywood to fit along the length of the threshold, so that we've got something to spread the mortar on when we set the stone. We measured the width of the threshold area at one end and took some measurements working towards the other end. We had to trim back the "temporary" plywood floor to make that section a little wider; it was about a 1/4" narrower than the kitchen floor area of the threshold. The circular saw made quick work of that.
Working fast, we marked up the plywood and cut it in two lengths. Everything fit right in.
We could have gone one step further by toe-nailing the last row of hardwood floor planks, since they're starting to separate, but I decided to hold off because I was starting to get a little tired and impatient, and that's not the time to try and pull a task like that off...too many things could go wrong. So, we'll get to that tonight or tomorrow.
Next up: cutting the slab!