Interesting. I used one of those little sunlite racks that mounted on canti bosses to carry a big handlebar bar that had all the mounting hardware and some of the stiffening removed. It sat there held in place only by it's shoulder strap. I had never heard of a rando bag and thought the whole thing was an original idea. I liked it well enough that I mounted the little racks on bikes that didn't have cantibosses using improvised brackets. If something like that is what a rando bag is, it was a handy setup. I don't see a reason it needs to be a special bag though. I used a big Nashbar handlebar bag with the hardware removed. It worked better on the rack than on the bar. It sat nicer than it hung, was in a better position, and the rack/bag combo weighed less than the bag and all the original hardware.

Edited to note that I never used my setup for randonneuring. I just used it for light touring, but I don't see why a similar setup wouldn't be fine for randonneuring For what little I know about randonneuring, I'd think it would be suitable for the range of what folks carry. At the lighter end of the range just the front bag and at the heavier end of the range with some kind of seat bag (maybe a carridice type bag). I'll bow to the knowledge of those who actually know something about randonneuring though.