In general, on US public lands such as the national forests, you are allowed to camp almost anywhere as long as it is not close to a highway or not interfering with other activities nearby. For ex., I have camped at the Shuksan Picnic site at the start of the Hannegan FS road, just before the last bridge on the way to Artist Point on SR542. Has a toilet, water, picnic tables.
There's also some spur roads south as you go east along this FS road that lead to the river and camp spots. And you can camp at the Goat Mtn trailhead parking lot, just past the TH sign.
On the south side of the bridge there's the Silver Fir CG (pay) on the west side, and on the east side a large parking lot which becomes a Sno-Park in winter. You can drive along this FS road and watch out for side roads going north to the North Fork Nooksack River for some great private free camping spots.
Lots of other opportunities for so-called commando camping. Try nwhikers.net for suggestions.