A couple more things I've found... this one from some Dutch forum I believe:
Apparently the author of whatever book that photo comes from thought 550 ft/s was appropriate.
As well, some similar bullet weights/charges
40 gr musket powder (FFg?) in the Springfield pistol-carbine gave about 600 ft/s with a 468 grain bullet (1860s ballistic pendulum).
30 gr FFG in the 1855 Harper's Ferry pistol-carbine gave 577 ft/s with a 525 grain bullet (modern chronograph).
However both of those have 12-inch barrels, and it's FFg instead of FFFg. In any case I guess somewhere from 500-600 ft/s sounds about right to me for the .577 from a revolver.
In "The Webley Story", Eley and Kynoch 15mm pinfire cartridges are said to only have 21 grains of powder, I suppose that could put them as low as 450-480 ft/s! However another source shows Eley loaded at least some of their 15mm pinfire with the same 28 grains of powder as the .577 cartridge.
Also I should mention the guy who owns the .577 mentioned in the first post said that he had reduced his loads to 600 ft/s for fear of damaging such a rare piece.