Let me deal with the point about separators. Separators, which are normally horizontal lines for menus and vertical lines for docks, act as visual clues for the boundaries of groups of similar items. There is less, or no, need for separators on non-linear docks since the items are off-set from each other vertically and/or horizontally - the visual clues are already there. Besides, if you really want to distinguish groups of similar items, place each group in its own Dock Folder.
But suppose there were separators in Circle Dock with user-defined images as you suggest. Now suppose that the image chosen was a standard straight line, have you considered how the image would look in the various positions around the Dock? It would need to be rotated such that its axis always points to the centre of the Dock.
And what happens if you decide to sort a level of the Dock - where do the separators go?
There are other implications you may not have thought about - such as the size of the separator object - same as a normal Dock Item or smaller?
You're asking me to add a layer of complexity which, frankly, has very limited usefulness and which will undoubtedly slow down the painting of the Dock. I really want to keep Circle Dock as lean and mean as possible.
Your request for named instances won't make it into version 1 the features of which are now frozen.