Hmm, interesting as that pin is listed as "Display voltage supply, positive, optional for radio module low version only" in my pin diagram, which could well mean that it will provide a positive voltage for a remote output, I would need to test it with a multimeter to be sure though.
This is a neat solution, if I had seen this adaptor before I fitted my RNS-510 I may well have used it rather than messing about with a Line Output Converter, although I am pleased with the results that solution is giving me now.