Pycairo has no support for creating Xlib surfaces. You can get a Xlib surface from PyGTK and then use Pycairo to draw on it, but there's no way to create one directly.
What I've done is make Pycairo aware of xpyb so it can creates directly an XCB surface from a XCB connection and a drawable.
As said in my mail to the XCB list, I'm now waiting for a review before pushing this upstream. :-)
For the first time, I guess, XCB has beaten Xlib support! ;-)