I did essentially what you are considering on my previous C28 and C320. I basically made the plumbing into an H with the left vertical being the salt water to head, and the right vertical being the sink drain to outlet. I added the horizontal line being the tee'd connection between the existing lines with an On/Off valve in the middle of the horizontal line of the H.
Now on my Nauticat even though I have a separate shower stall and faucet I also have a (second) hand-held shower type faucet at the sink which is right next to the head bowl. Now I just use that hand-held to add, and more importantly Pressure Spray, the exact amount of water I need to clean things up. My point is if you have a shower head fixture, as many boats do, within reach of your head bowl I have found that to work much better than simply to have it flooded with fresh water.
Wickford/Narragansett Bay RI