i would never do it, i don't think, but the founder of Clif Bars has an 8 pound touring kit he packs into a supersized under the seat bag.... it was outlined in an Outside magazine a few years ago....

something like extra jersey, microfit jacket, nylon off the bike pants, lil' hat, extra socks, patchkit, minitool and a credit card were pretty much it i think.

i go light, still manage to pack a full five bag load - but lightly.

If you can grovel, sleep in ditches, or have previous experience dirtbagging it, you'll do better on the road going ultralight. learning how to identify a warm patch of shrubbery is key.