First, let me thank everyone at Tom Bihn for having a shorter person model the Synapse as well as a taller person. It really allows me to have a good idea about the size of the bag. It looks perfect for my height.
Second, I would like to suggest that a loop as simple as one strip of webbing sewed flat to the very back of the bag. I use my current backpack (which is falling apart so I'm looking for a replacement) when I ride my bicycle to and from work each day. I have my red blinking light attached to the back of my bag and love having it there. I believe that this little extra flat loop on the back of the bag would not deter from its chic look at all and would give me and possibly other commuters a place to attach their bike lights.
I've noticed that many of the other backpacks (Brain Bag, Smart Alec, and Ruck's Sack) have a place on them where this can be done.