I just got done bathing one of the dogs, and I feel your pain! For a dog bath the heck with the inspector (he whispered). Lets think of it as recycling. What the yard giveth, the yard taketh away. So be it. I think your idea is great. For extra freeze protection, trace your water lines with heat tape and insulate. Also, this is an excellent opportunity to reinvent the cistern. Basically a rock filled pit that allows the dot wash water to dissipate into the soil. Your dry-well is the same idea.
Being that I am currently damp and have finished chasing the wet dog around the house with a towel, whilst the dog has rubbed all the water off on walls and furniture, I feel qualified to say, "Way to Go". Great plan, have fun.