Fresh water source for head flush?

[sailmed]

This may have been addressed elsewhere, but I haven't found it in my search. 
There seems to plenty of room under the sink to place a nice Plastimo flexible bladder to hold fresh water to flush the head, so as to avoid those micro-critters from Bay water dying in the lines and causing that unpleasant odor that plagues many boats.  Has anyone done this? If so, how have you done it?  Has anyone gone as far as to install a deck fitting for filling?  I know I can use the sink water route, but it's too contrived to explain to guests.

[Ken Juul]

You only need to worry about the critters when you don't flush for an extended period.  If you use the toilet daily they don't have a chance to get smelly.  I went with replumbing the head intake to the sink drain.  Flush the line with fresh water from the sink on close up.  No smells on the first flush the next time at the boat.  To prevent sucking air from the sink I use an oversize drain plug.  Old fashioned white rubber one with a lip around the plug part to give it a good seal.