Okay - I just buzzed out the cable. It is wired properly (i.e.
DMS59:VGA2_RED -> VGA#2:RED, etc). So my money is on the nouveau driver
being confused. I'm going to post that issue on the devel list.
In the meantime, I'm still interested in the approved manner of tweaking
XRandR setting while the GDM login screen is being displayed.
Regards
-- Charlie