Following up on myself... :-)
In short: the server's GLX module tries glxdri, but the latter thinks that the intel driver is not DRI capable since it uses DRI2 and never calls DRIOpenDRMMaster().
^^^^^^^^^^^^^^^^ This should have been DRIScreenInit().
I tried to add glxdri2 to src/external/mit/xorg/server/xorg-server/glx/Makefile.glx
For the record, support for GLX DRI2 is now in -current. Grégoire