lorax

Building on what Chong said, if you're serious about using your bathwater for irrigation, a catchment barrel makes really good sense. You can also then put a small passive filtration system on the line, which should eliminate some of the chemicals and stuff that you're concerned about. In Ecuador, the most common system is an inline three, with sand/zeolite in the first stage, ceramic in the second, and activated charcoal in the third. You probably will need a pump, though, if you're going to level or above grade from the tub. It's not that difficult to divert the drainline, though.

The house I'm currently designing incorporates greywater catchment and filtration (for irrigation use) from the bathrooms, and greywater settling and organic reclamation from the kitchen via planted filtration pools (the pool in the last step serves for growing Tilapia and rice). If you're serious about retrofitting, I can send you some drawings and possible ways to go about it if you'd like.