I think that
I don't think that Assange would have the high ground in this hypothetical case [...] Clearly, it can't just be that transparency is always a morally superior end state [...]
is conjectural, and probably unproveable. I may well be that transparency always gives one the moral high ground. Of course, that's more or less orthogonal to the degree of umbrage taken (is umbrage taken in degrees?) by John Q. Public. Nobody likes their laundry aired by other people, regardless of how clean it is...even as people put more and more of their lives online for all to see, I suspect that we all want to be the people who control the flow on our personal information pipes.
I wonder whether there isn't a threshold to be (admittedly, somewhat arbitrarily) drawn in terms of "degree of influence". What an average member of the public chooses to divulge or keep secret affects a (comparatively) small group of people. Conversely, the machinery of international diplomacy, or the military, or corporate greed/corruption, etc. are---rightly or wrongly---perceived to affect a much wider group of people...potentially spanning multiple nations. It seems to me that this is the yardstick by which we (implicitly) judge the rightness/wrongness of divulging information. The greater the number of people affected, the more we want transparency (subject to my earlier caveat that agents seek to retain control over their information pipes, where e.g. the military, or Mega-Company Inc. can plausibly be construed as agents).
Hmm. That reads a bit ranty and disorganized. Just fired this off without much thought...