There is another shower underneath with easily removable PVC ceiling and access to the pipe. So indeed theoretically I could put the valve in the shower ceiling of the lower apartment. But that segment of pipe is galvanized steel, so I would have to cut it and add threads. I actually have the ratcheting tool to do that but I would still need to buy the threading die. It’d be a bit weird if the lower apartment had the control over the water above. Not a show stopper but I think there are better options.