Hardly difficult and definitely not impossible... I had no trouble fitting a Carradice Nelson Longflap to my 54cm carbon race bike using a Bagman QR support. The only limitation is that you need some space between the saddle rails and the tire. If you've got a small frame or very little seatpost showing, you'll need to use a smaller bag (e.g. Lowsaddle Longflap, Barley, etc). The OP isn't planning to take much stuff to begin with, so one of these smaller bags would likely be perfect for him.

FWIW, when I was thinking about using my race bike for touring I planned to swap the expensive carbon seatpost for a cheap aluminum one. I, honestly, can't tell much difference between the two and didn't want to risk damaging the carbon post.