Thanks ContinentalFan for reviving this thread.
It would be very nice if CO could offer some more transparency on this issue.

FWIW, the 3 or 4 times I've tried to waitlist on CO's 46/47 (IAH-AMS-IAH) I've been hit with the "waitlist is closed" on every occasion. And I've typically been requesting the upgrade 3-4 weeks out from the flight dates. I guess a combo of popular route/high loadfactor in FC yields this result...