I'm sure it's possible to get a 20lb cutbow. Ive caught small ones in the Tayloe river so there's no reason one couldn't be sitting below the dam gorging itself on mysis. Good question though. I would think a 20lb cutbow would have to be in the book somehow.
Actually we have hooked into several cuttbows in the Colorado River that were over 20 pounds. The reason the rainbow record stands at only 19 pounds is because many people catch larger rainbows than that but almost all of them are cuttbows, so they are disqualified. So in the Colorado River near Glenwood, The Taylor River below the dam, The Pan below Ruedi, and the Blue below Dillon all have fish over 20 pounds, but pretty much all of them are cuttbows so they do not qualify.
The official Cutthroat Record is 17 pounds and was a yellowfin from Twin Lakes.
The best place for a pure large rainbow to beat the record is the Gunnison Gorge. They are almost all pure 'bows, and bows over 10 pounds are common in the remote holes, but extremely hard to catch and land in the powerful current. They sit on the bottom of holes that can be deeper than 40 feet and are boiling with current from hell. Very hard to trigger, almost impossible to land. Have hooked many of them and they go bye bye quick. It's pretty much all big lures for them, forget flies unless a massive hatch is going on.