While I understand that the question and discussion is about 500mm mirror lenses for Nikons am I wrong in thinking that the goal is for photographing a distant subject and having it fill the frame? I don't know what your current longest lens is but have you considered a high quality 2x. The reason for this comment is that I have a Mutar (Blad) 2x that I use with the 150, 250 and 350 and get excellent results of course with a tripod and the loss of two stops neither of which bothers me. The 2x retains the minimum focus of the lenses. Just a thought.
http://www.jeffreyglasser.com/
http://www.jeffreyglasser.com/