You know, now that I re-read your post, I wonder if you aren't thinking about a different product than we are.
A safety sausage is a small inflatable tube around 4' and a few inches wide, that rolls up into a nice, compact little 3"x2" roll, and clips nicely from any available D-ring.
Compare that to the SMB (surface marker buoy), which is generally about 6' tall and 6-8" wide when inflated, and when deflated and rolled it's still a fairly substantial package that's a tube 6-8" long and 2-3" diameter... far larger than you'd probably want hanging freely from a D ring. One of the more common methods of stowing them involves a couple loops of bungee or tubing on your backplate, and stowing it along the side or bottom of the plate, where it's mostly out of the way.