I didn't even know there was a pier but I would guess that most people use it for crabbing. If I recall correctly, halibut tend to like the open ocean more than big harbors like Tillamook. It's worth a try, but I would be surprised if you catch something other than a flounder and sculpins, with the occasional bait thieving crab. Oh, and I suppose there's also a remote possibility of hooking up with a salmonid while they're en route to spawn in the Wilson.
I typically fish from the south jetty in Barview which is few miles north of Garibaldi. If you feel comfortable scrambling around on large boulders while keeping a watchful eye for sneaker waves, you can catch greenlings, perch, rockfish, cabazon, and (less commonly) ling cods. The waves can get pretty hairy the further out you go, but I find them easy enough to manage within a 100 yard stretch of the CG tower.
What sort of clams did you find out there? I drove home during a little minus tide and was surprised by how much muddy substrate was exposed.
Cheers,
Chris