Tee the head intake line into the head sink drain line just above the thru-hull. Flush with sea water normally...close the seacock and fill the sink with clean fresh water to rinse all the sea water out of the system. Or keep the thru-hull closed and put water in the sink each time to flush.

Better yet, replace the toilet with one designed to use pressurized fresh water. The Raritan SeaEra http://www.raritaneng.com/products/toilets/electric/sea_era.html is available in both sea water and fresh water versions, and both versions are available as a "conversion" (everything south of the bowl) http://www.raritaneng.com/products/toilets/electric/sea_era_conversion_kit.html . Price is around $350--which isn't a whole lot more than a separate flush water tank would cost you including the separate fill, vent and plumbing required (you can't connect it to your potable water plumbing)--if you shop the right retailers.. I think Defender has it.