Pannier for cannondale caad9?
Heelstrike, which Neil Fein mentioned in a comment, would be my chief concern. You'd need a rack that moves the panniers aft. Axiom makes just such a rack that is designed for bikes without rack-mounting eyelets—instead it mounts to your quick-release skewer (which could make changing flats a PITA). I have seen people touring on road bikes with these, so it can be done. It would also be possible with some racks to mount the rack to P-clamps at the bottom. They also make this clamp-on seatpost rack that supposedly has a 15-kg capacity. Should be more than enough.

You might also consider a large seatbag. This one might be too small and is spendy, but it seems like it's in the right direction (and if you haven't explored Rivendell's website, do so—neat stuff).