We generally used the comfort station in the 400 loop when staying at that end of the 600 loop. The 600 loop is very small, and during a prior trip we were at the other end, closest to the 800 loop and we usually used that comfort station.
While the 600 does not have its own comfort station, I think that you are just as close or closer to one in that loop as in many of the sites in larger loops.