That will work as long as there is enough water in the lines for the pump to move through the shower to the FW tank. As soon as there is air in there (if you ran it dry) it will stop drawing from the supplemental tank.
Still it is a great idea for boondockers. I could easily see formalizing that process path in the plumbing cubby of my 2506S with a valve or two to engage the path.
It would also be a neat project for some 24V water valves and an arduino controller and a tablet. Thanks for destroying any free time I may have for the next couple of months