Dear Incognitus:
> .... For their mitre-wearing proclivities,
> there is photographic evidence.
I can only say that, in a year on Athos, I never saw such a thing. Maybe I was never in the right place at the right time. I can think of four theories to explain this:
1) I was wrong.
2) There is typically a bishop resident on Athos these days, and somehow he is confused in the photograph with the Proestos.
3) The Proestos at the time was a Russian, and the Epistasia let him wear his mitre.
4) Sometimes, on great feasts, a Greek priest may wear a kamilavka that is so ornate that it could be confused with a mitre.
Wishing you and all a joyous Pascha,
Photius, Reader and sinner