I did something like this in reverse for my head in my Viking 33. Keep in mind I am in Lake Ontario and therefore flush relatively clean fresh water already.
Basically, this boat is cleared out for racing and daysailing. I got rid of the evil old 10 gallon tanks in the galley and the head, and the Whale pumps were half dead anyway. People can wash their hands with "city water" brought aboard in jugs...they just have to soap up in the basin and then rinse with a "gravity bag". No different from camping, really...except the head is in full working order!
Anyway, I "stacked" the drain and the head water intake as follows:
Seacock---short length of double clamped hose---plastic T-fitting to head---short length of double clamped hose---plastic stopcock---short length of double clamped hose to sink drain SS outlet.
Method if you want to flush: Open seacock, close sink stopcock. Pump away.
Method for wash-up: Fill sink, prior or after, wash up, and either let greywater go straight out, or close seacock and pump greywater through the head.
Method for winterizing head, sink, etc. Close seacock, fill sink with pink stuff, open plastic stopcock, pump system. Slosh pink stuff in head bowl, shut 'er down.
I use the same "divert antifreeze from sink" method to winterize my engine and Lavac head on my steel boat using a simple bypass from the drain to the standpipe, which I seal from the outside to make the standpipe into a nice tall reservoir for antifreeze.
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