I am also considering the "running commute" on weekends that I work, because the traverse is over such a hilly neighborhood that I'd probably be faster on foot than bike- and safer too, with traffic. I have the fortune of having showers and lockers at work, so I can stash my stuff and not have to carry much.
However, on days where I might need to carry something in, I do have a camelbak pack that isn't too obtrusive. I'll give it a try next weekend!