Use gray to flush black

I would suggest that you find a flat pan and collect the shower water as you stand in the pan. Some might miss it's mark, but pour that into buckets, maybe through a strainer to collect the hair. There is small lines and valves that water has to flow through, and the use of gray water as a flusher could lead to a lot of problems if something that went into a gray tank gets stuck along it's path, or causes gate valve problems in the toilet.