I realize we cannot add a RadSeparator to the RadRibbonBarBackStageView currently (it's not a RadItem) but there really should be some kind of "Separator" for the navigation (we have separators for every other navigational elements: ribbons, groups, menus.)
I also realize that earlier versions of the Office Backstage did not include a Separator however, all of the newer versions do so no harm in allowing someone to select "Windows 7" as their theme and still allow this, is there?
Adjustable Properties should be (IMHO)
EdgeWidth - this is how close to either edge the line will be drawn (there's no reason the line has to go 100% across)
LineHeight - in pixels how thick to draw the line. set limits at something reasonable like 1-10
LineColor - obvious
Height - users should be allowed to adjust how tall/short the separator is. This is not to be confused with LineHeight. This is the total height of the control whereas LineHeight is just the line being drawn.
The attached image shows what I came up with in my ap - I got a little fancy with the line itself by allowing a leading and trailing linear gradient and my usercontrol lets me select the length of the gradiation but no need to go totally cray cray.
Anyway just my two cents :)