I can't speak for perception in diplomatic circles but can give you a view of "rank" from the corporate world when it is required to interface with the military. Firstly, whatever rank in whichever force normally receives an enormous amount of respect.
That said, at the higher levels of corporate life where life it is all about business and ROI, seniority is required for retired officers who wish to work as consultants so they can provide the correct level of communication with MoD etc.
The consultant working for me some years ago was a 1 Star (Brig retd.) and excellent he was too. Quite frankly, any rank under that would not open doors and it was obvious that this was reciprocated by serving officers in various branches. We met some excellent Lt Cols., Majors, Acdre's etc but their experience and authority at that level was very much within strict parameters of knowledge. If nothing has changed since, then I'm not sure how "de-ranking" will work in diplomatic circles, unless all countries follow the same path. It certainly wouldn't work in the corporate field.