The xf86-video-modesetting 0.6.0 driver has been released with support for virtual connectors and a couple of bug-fixes.
The patch for supporting virtual connectors is quite small as are the other changes that make up this release. Since xf86-video-modesetting 0.5.0, the only other changes are removing alloca usage again, fixing a crash with a udev race condition, eliminating a miInitializeBackingStore()
call, and putting the m4 macros within the m4 sub-directory.
The xf86-video-modesetting 0.6.0 release announcement can be found on the X.Org mailing list