Fheardhaigh, if you figure something out for the pannier, or, in particular the Arkel Briefcase, I'd be very interested in hearing about it.
I currently commute on a beater hybrid, my Arkel Briefcase is my work bag. I lug my laptop, lunch, and sometimes clothes around. I use the Briefcase even when I'm not commuting by bike (winter foulness and dress slacks for work don't always mix too well). I'm very eager to add a folder to my arsenal, a Downtube is high on my list of starter folders. I'd love to be able to commute on it but, well, if I can't use my Arkel Briefcase the Downtube wouldn't get nearly as much use as I'd like...