Yeah, I've only been boating for 20 years and I've only done Westwater about 70 times, so I guess I should keep my trap shut. Thanks for setting me straight.
Ice does not form well in moving water. Molecules hitting each other at greater speed generate kinetic energy, which makes it harder for them to bond to one another in the freezing process. Also, minerals, chemicals and dissolved solids create more bonding problems in moving water. The reason why it freezes down near Moab because the slack water in the spots where there isn't any gradient creates a better opportunity for water molecules to bond.
So, by examining our little physics theory, it's more likely that there will be ice forming in the stretches above and below the canyon (all things being equal; average temps, etc). Erring on the side of caution (in this case) may in fact put your raft in a little more jeopardy than a canyon run. If you were planning a mid-winter descent instead, I would agree that rafting that stretch without confirmation wouldn't be prudent. But it hasn't been that cold on the Western Slope this fall, and I have been down the canyon 2-3 times in December, so I still believe that it would be clear for boaters.
I recall there being ice on the sides of the riverbanks in February 1995 in the canyon, and seeing the river iced over near Moab as I drove home to Durango. Granted, I was kayaking the river that time, so we had a somewhat easier option if we encountered problems.
Hope you were able to get on the river.