It's been a number of years since I've run the Encampment. I would grade Upper Damnation Alley as Class V at moderate flows and V+ at high to insane flows such as I had when I ran it the first time in late June of 1978 at over 2000cfs. The trees along the banks, then, were submerged in over 2 feet of water in many places (that means, few to no safe eddies). I've also run it in lower flows as well where UDA is a creeky 4.
I've done some rafting as well in 14 to 18 ft rafts, but only on other larger rivers (Ark,MF Salmon,Grand Canyon).
I think you might have a difficult time rafting this river for different reasons at different flows. Low flows would mean a lot of dragging and tight squeezes for your raft in the first mile or so with tight boulder infested drops in the meat, and high flows would mean difficulty in maintaining control in long, tight, pushy, rocky drops in the Damnation Alley area, with swimmers being a potential nightmare. Moderate flows would be your best bet and it would not hurt to be on top of your game the day you try it.
Logs in a small stream bed environment are always a concern. Each year brings new possibilities as you probably are already aware of.
Best of luck!