If you are not going to use the toilet for its' normal purpose, then I would just re-name the tank the "gray water tank" and dump away! :) I don't see any difference between the gray and black water tanks except their name.
Every campground I have been to has either dump facilities at the site or a separate dump station near the exit. None of them has ever had a separate dump station for gray water that could be accessed by a vehicle. I have seen gray water dump stations in tent camping areas as has been already mentioned, but you had to tote the water there to dump it.
My feeling is that you are overthinking this situation - unless I just don't understand the problem.
Barney