There are plenty of ways to get a rack on a bike not made for it. I don't tend to think a rack is all that much of a burden when riding with nothing on it. I suppose if you are really racing hard and need to shed every bit of weight (I don't think most racks would give you an aerodynamic penalty).

http://www.axiomgear.com/products/ge...eamliner-road/

That one from axiom is a slim rack made to fit on a bike with no braze-ons/taps. I do not know how much weight you need to carry, but it should do the trick and I doubt you will notice it while riding. If you do, you will just need to get 640g faster...

The rest of the commuting gear you might need (lights, locks, bells) can all be easily taken off the bike when you don't need them, and of course you do not need to train with any luggage on your rack.