Cool! I like your chalkbag idea. Would have been perfect last week when I was trying to eat cashews but couldn't get my gloved hand into my pocket very easily.
I fastened a small Crumpler bag to my handlebars to store my camera in easy reach. The bag, a John Thursday 80, has a velcro belt-type strap that goes around the bar, along with a velcro closure but it is still quick to get out. It's not waterproof, however, so the camera goes into a sturdy zippper bag when there is a risk of rain. . . which is most of the time.
edited to add: It's not "on-the-fly" shooting, however. I do have to stop riding to take any photos.