We now also have gallium packages in the main xorg-edgers PPA. The way it works here is a bit different than the radeon gallium PPA: The normal libgl1-mesa-dri contains only the classic mesa drivers. The libgl1-mesa-dri-gallium is an extra, optional package with gallium drivers. The libGL search path is set up so that the gallium drivers are loaded if the -gallium package is installed (Robert's clever idea).
Note that the -gallium package in xorg-edgers also ship nouveau and intel gallium drivers.