I use a rack. Actually, I have a full carbon road bike, and it doesn't have a rack - and it's a real pain. I keep using my commuter bike with a rack for riding because of it. Need to bring a light rainjacket in case it rains? - oh, right, now rack. Need bring an undershirt because I'm biking and the sun will go down, or want to sit at the lake for an hour and won't be warm? No rack. Etc etc...I recently discovered my carbon bike was to small, and am looking for a new one in a larger frame - but some sort of rack is going on there. It's to useless without one.

The closest thing I know of that meets your requirements is my dad has something like this -

TopeakŪ Cycling Accessories ? Products - DynaPack?


His doesn't have such a big attachment thing, it looks a little nicer, but either way you can take the "bag" on and off fairly easily, leaving just the seatpost mount attached more long term to the bike.

Another thing to consider is how much you can just leave at your job - I have a desk at work, and I'm a guy, so I leave an entire change of clothes their and don't carry clothes back and forth on my bike. Periodically I'll take them home in my car (since I don't bike all the time) and wash them (usually for convenience I'll bring in the clean set and bring home the dirty set so there's never ever a time I don't have clothing at work).