As far as I could find, backwoods logging roads is all you get, unless you leave the area.
I've been sporatically shooting in the Corvallis area for about a year and a half now, and have found absolutely nothing. I've called the Benton County Parks and Recreation Services Department and all they had to offer was "there's a no-projectile ordinance in Benton County, so it's best to find a logging road and just find a place to shoot."
He also mentioned a road (can't remember the name, but I can show it to you) off HWY 20 between Albany and Corvallis where people sometimes go to plink, but loud guns are generally a no-no there, as if it gets too raucus the locals might call the cops and get you kicked out.
I've meant to, but kept forgetting to call Linn County and ask them about shooting nearer the Lebanon area, since they're on the more conservative side of I-5.