I have a small drill-driven pump that has a hose adaptor on it, but there are quite a few available for this kind of thing (look up pumps at someplace like Ben Meadows). One bathroom has a window and I would probably put a tank on a stand outside that room and let gravity feed to the adjacent garden when needed. It would need regular cleaning with shampoo and soap in it, to keep the tank from getting too crusty, but shampoo and soap aren't going to hurt the garden.

SRS