I have never caught anything from Coalbank Slough, but I have caught salmon from Isthmus Slough. I have no idea if salmon travel up Coalbank, but you are pretty darn close to the confluence with the two. On Isthmus I have been successful by casting both spinners and eggs under a bobber. My favorite lure was an old garage sale special large rooster tail in a chartreuse color and silver blade. I couldn't tell you what size of lure it actually was. I have had a couple of salmon on large blade Blue Fox's of various colors as well.
I have never personally caught one, but my Dad tells stories of hooking into striped bass on the Isthmus when he was younger. He was up stream farther though in some of the marsh grass. He used to take a fishing pole with him, and bobber fish, when he would duck hunt up there. As far as I know it is just a fish story though
There are also a couple of places to bank fish along the Coos River Road as well. Good luck, I hope you end up with something.