I have never fished the Powell, but from what I've read about it, it does has lots of smallmouth and rockbass. I think that since it is located in a more remote area, it receives a lot less pressure, and I'm sure not many fly fishers.
TWRA does have one boat access point on the river and I read that most bridge crossings and some roads along the river, have access pullouts.
since it is a free flowing river , it is controlled by the weather. a hard rain upstream would no doubt blow it out for several days.
There are a few guys (non- fly fisherman) on east tn fishing forum that occasionaly post a report on the Powell.
I would take the usual flies and a 5- 8wt rod., bass leader and 2x tippet, black , yellow, and white poppers in size 4, some green/white and white/white clousers, weighted buggers in black, brown, olive and maybe some zuddlers for something a little different.
This is one of those east tn rivers I want to eventually float and fish myself. good luck and keep us posted how you do.