I've floated the Colorado from Two Rivers park in Glenwood Springs to South Canyon in January. IIRC, that was after ruling out a couple of runs upstream of Dotsero and the Grizzley Creek stretch due to iced over conditions.
One thing to remember is that even on beautiful sunny days when it looks like everything's a go, there can still be ice dams in places that are continually shaded. It's really important to thoroughly scout the shady bends before launching.
Also as for Westwater and R-HT, the guideline is that if there's ice in Debeque Canyon, there'll be ice downstream on those runs. Consider the consequences of coming around a bend and seeing ice across the river, and having the banks iced up and impossible to climb without crampons and an ice axe...
Looking forward to hearing what others say, and give us a trip report if you go!
-AH