A little more of a PITA than than Plan A, but not much ... You will need a couple of new components: a y-valve (even a 3.95 garden hose y-valve will work for this), a gallon milk jug and a couple of feet of 3/4" hose. Put the y-valve into the head intake line...connect the short piece of hose to the other side..fill the milk jug with clean fresh water..stick the other end of the short piece of hose into the milk jug...flush the toilet. Stop flushing when the milk jug is empty.

Whoo hoo