I'm rather fond of the Topeak line of bags and have a small bag I'll put on my road bike when I want to carry the camera. A large handle bar bag will affect steering, so keep the bag as small as you can. Don't forget, your rain jacket will probably fit in your back pocket.

Can you temporarily put a aluminum seatpost in your bike? My road bike is aluminum so it wasn't an issue to swap the carbon seat post for an aluminum. That way, when I really need to carry stuff I can put on a cantilevered rack and bag. I don't know if that is an option for a carbon frame, though.