What I was thinking is the freshwater system now has a pump... I would only be adding a saltwater pump, a y valve and a hosebib/enclosure.

I am assuming you can "throttle" the saltwater system to deliver low volume when needed... for the "people spray" and yes, the spray head would have to change... but I've seen rotating spray heads that give you the option of gentle shower to hard core jet.

I would want to simply choose fresh or salt as desired. (and of course have the option for hot water too...)

As I put this together in my mind, I am beginning to think that all the valves make it not quite as "clean" an installation as I would really like... and that different hosebibs in different locations might be the better answer. ("no, no, dear... that's the salt water wash..." )

I was toying with this whole concept as the galley offers me the option of fresh or salt, depending on what foot pump I use... and I was wondering why a washdown system tied to the boat's freshwater (shower) system couldn't give the same salt/fresh options for a cockpit shower. Of course the galley system uses two totally separated systems... including spouts... so it really isn't a good example.