I think, why not? I do it myself - not on a Roubaix, but on a Giant SCR C4, the predecessor of the Defy Advanced, a carbon road bike with similar geometry to the Roubaix. I use two Carradice bags - a Nelson longflap and, if needed, a Barley as a bar bag. I don't bother lowering the gearing, but then again, I was already running a compact with a 12/27 on the back, so I can get up most things.

I'm not as lightweight a packer as you, so I keep this for credit card tours on which I don't need a tent. It's fun though, and means I have a pretty much full-on road bike to play with if I want to stay in one place for more than one night.