I think this behaviour(carrying infants on the back almost from birth) is a product of captive life- I have other females(or photos of them) in zoos doing it also. Perhaps its easier for the mother. In the wild I doubt it happens- a tiny baby could easily fall or be brushed off as the group travels in undergrowth- these dangers don't exist in zoos.
Good point, maybe higher ranking females (like Frala and Shinda) demonstrate this behaviour as a sign of confidence. The offspring of a lower ranking female may be at greater risk of injury by being grabbed by another gorilla (something they wouldn't attempt with the offspring of a higher ranking female).
I think I recall you mentioning Kriba's daughter, Joas (1989), does this with her offspring too, despite Kriba never doing it. Kriba, while middle to high ranking in Kibabu's troop, has always been a nervous individual and distrustful of Frala's adolsecent sons in particular.