I have been exchanging notes with a Byzantine art historian who recently found this painting although she has never actually seen it. Fundamentally unknown, it was sold at Christie's in 1995 as a portrait of John VIII, and then disappeared into some collector's private world. Christie's dated it to the early 1500s, maybe as late as the 1520s. The few scholars who have mentioned it assign it to a school or follower of Gentile Bellini.
When I saw the picture, I immediately saw it as Mehmed II. Mehmed -- and later Suleiman the Magnificent -- were sometimes portrayed as Byzantines, which is, I think, shorthand for "the ruler in Constantinople."
Here, for example is a woodcut of John VIII serving as a representation of Mehmed II in the 1493 Nuremburg Chronicle.
She -- the art historian -- believes the painting is John VIII, and feels details closely resemble those of John in the Sinaii portrait:
One problem with either identification is the late date of the portrait. Mehmed died in 1481, John in 1448. Was someone making a collection of Byzantine emperors or Ottoman sultans?
The main problem, though, is that the location of the portrait is unknown. I am writing this entry on the off-chance that someone our there reading has seen the portrait and can give more information about it. Where is it? Who was the painter? Are there similar paintings out there? Would the collector make himself known to the art historian and allow her to see it?
You can write me at the e-mail address up at the right.