Unfortunately, I've had no luck there. I have a separate furnace/ac system for the upstairs. The equipment for the bottom 2 floors is in the basement, and the equipment for top floor is in the attic. The vents for the lower system are in the drop ceiling of the room with the equipment. AFAIK, there is nothing that runs from the basement to the top floor (HVAC-wise). Plumbing hasn't been much help yet either, but I will keep looking.
I may just pull the baseboard and carpet and run the cable a few inches out from the wall and through a hold in the floor to take advantage of what I've already cut. At least I know the drop will be in the right location.