If you really need it for multi modal, I wonder if you can go to a folding specialist store and see if there's a way to fit the Tern Trolley rack onto the bike. That item makes a 20 inch folder as convenient to wheel around as any brompton, if not easier. It would be as easy a rolling around any of those laptop bags. Don't try to use a shoulder strap for anything far, carrying a 20 inch folder really SUCKS! you'll think it's fine until you do it. Heck any folder over 20 lbs would be tough to lug around! For me it's just to the elevator and to my desk so I just grab it and go. But if I had to take the underground tubes and still wanted a 20 inch folder without going 16 inch, I would have bought the trolley rack for my Tern for sure.
http://www.youtube.com/watch?v=SuZpMC5_fMI