The limited amount of randonneuring that I have done has convinced me that there is no such thing as a rando bag, as I have not seen anything that was consistently used by more than a small minority of riders on a brevet. On the randonneuring forum, some have referred to a small Carradice style saddle bag as a randonneuring bag but I do not think that is what you meant.

I have occasionally seen small platform front racks that were called randonneur racks and a variety of bags on those racks, sometimes called handlebar bags that were not actually hanging from the handlebars. Is that what you meant by rando bag? I think over the years that Velo Orange has made a bag they called a Randonneur bag for such racks.

Another option you might consider is a handlebar bag like an Ortlieb that is mounted lower on a second stem with a handlebar stub. It is closer to the location of the rack mounted front bags on those small platform racks. I use something like that for touring and I like the ease of bag removal from the bracket to bring the bag into stores and restaurants, I have several such bags that are not Ortlieb but my bags are no longer made.

I have photos of several of my bikes with second stem used this way at this post:
Racks/Bags and Interrupter Levers

I recognize that the second stem is not a "rando" bag system, but I am not 100 percent what you meant by rando bags. I am just guessing you meant a bag on a small front platform rack.