I'm hopeful to design a shower that can allow for grey water use in my adjacent garden. The problem is I need to find a diverter that can be readily accessible for use. My plumber has shown me that because the drain is about 15-20" below the floor level, it will be necessary to have the diverter close to the "y" at this point. That leaves me no easy way to get to it (the diverter) other than a trap door in the floor of the adjacent closet. I'd like to have the diverter work and look like those in a tub/shower. Any options?

You're asking for a diverter that works on drainage that looks and acts like the diverter on a three way tub/shower valve or a diverter spout. Sorry they don't make a fitting like that or if they do I haven't run across it yet.
However consider a wye, a gate valve and a trap door in your closet. Good luck, Tom