I've actually been thinking the last few months about this too. I want a bag for overnight/weekend jaunts, just enough to carry a change of underwear, a jacket, shoes and a toothbrush. I too have a carbon seatpost and I also happend to hate the look of those racks that clamp onto the posts and stick out into thin air (I know, I know, function over form, but still...)

The Carradice's looked liked a good solution to me, too, although unless you buy (or fabricate?) hook adaptors, they won't attach to non-Brooks saddles. Also, because of the width of the bag, I do think that the back of your legs will bang against it if it's up against the seatpost. The Bagman Q/R frame solves both these issues, but it can cost as much as the bag itself and I've read a lot of reports about it being overly fragile.

Anyway, I'm still searching--please let us know what you do in the end!