750's need the oil bag because they run on a "dry sump" oil system, meaning that the bulk of the oil is not kept in the oil pan. I can't speak for the Honda engineers, but I'm guessing that a large part of the reason for this was to increase the ground clearance from the bottom of the engine, which is the oil pan. If the 750 ran a wet sump system then the oil pan would be much lower and would more easily get hit by various road hazards.
I'm sure you could work a separate oil reservoir into your 550, but I don't see much of a benefit that would be gained by doing so.
