I would imagine that a wall mount urinal wouldn't need as large a water supply pipe as a wall mount toilet, since it's only flushing away liquids. Also most modern wall mount urinals I've used seem to have rather anemic flush flow anyway. Also a small tank sufficient for flushing a urinal could easily be incorporated into a one piece wall urinal setup.
For a wall mount toilet, as previously mentioned, a separate wall mount tank would probably be best. It could also provide extra gravity force if it's mounted a few feet above the commode. I remember seeing photos of really old indoor toilets with the tanks mounted way up high, and the flush activated by a pull chain.
I also thought that the existing exterior wall could be doubled up with 2x6's to provide not only the extra rigidity but also the weird spacing (17" OC, really?) and room for the DWV piping. I'd however be a little concerned that the DWV pipe might still need to cut into the floor sill plate, so that might need to be reinforced at that spot to compensate.