Dipnetting is always worthwhile
Dipnetting is a very unique adventure. If you have an opportunity to do this it is lots of fun. Without an ATV, you are either limited to walking or paying $100+ each for a charter. From O'Brien creek you can walk out the trail to some good fishing locations. I usually don't go down the trail more than 3 miles. There are lots of people who take mountain bikes on the trails. You could also take a game cart to haul your gear and fish in. Or like you mentioned take a good pack frame and haul everything on your back.
The basic gear you will need is a dipnet and the personal use dipnet permit. I have the black mesh net with a fiberglass handle and a D-handle on the end. The handle is 10 or 12 feet long. Take about 100 feet of rope to tie yourself off to something on shore because if you fall in your chances of getting out are slim. I leave my coolers near my vehicle. I have a couple 70+ quart sized coolers. If you are hauling fish in a back take a couple trash compactor bags to line your pack with so you don't have slime running down your back while packing your fish out. Take some smaller rope to tie your fish onto so they can stay cool in the river while you are fishing.
If you head out the trail you will see places where people/animals are going down to the river. Follow these down to the river and look for a back eddy( a place where the current is hitting an obstruction and heading back up river.) The salmon use these to help fight their way up the river. In the narrow parts of the canyon the salmon will be close to the shore of the river. Find a good back eddy and put your net in it. I have caught fish in less than a foot of water before. I have also caught them in deeper areas. The current is extremely strong so it might be difficult to hold your net in the back eddy. If you can not find a back eddy, you will have to sweep your net with the current. If you feel something bump into your net, pull it up out of the water because it will be a salmon. Some salmon hit your net like a mack truck while others hit it very lightly. If anything feels out of the ordinary pull your net up and check for a fish.
One spot I always catch fish is just past the tunnel. At the very end of the tunnel go down the rock slide and start fishing. Good luck and let me know if you need any other info.
Thanks for the info. I will let you know how it goes.