This is a great resource for finding a date to volunteer at trail days with Washington Trail Association (WTA)
I made this video about trail maintenance opportunities on the PCT for National Trails Day and there's still some work day for August/September that are open to join! If you love Mount Rainier, car camping, and trail maintenance, this may be the perfect way to spend your weekend.
While not explicitly in the PNW, the Great Divide Trail Association is always looking for help on its trail-building trips. Most of the trips this coming year will be in the newest section of trail near the town of Blairmore, AB. Nearly 100km of trail has been built in the past few years and this section of trail doesn't exist in any park so the Great Divide Trail is entirely dependent upon volunteers to help build, protect and advocate on behalf of the trail in the pursuit of national recognition. If anyone is interested feel free to check out greatdividetrail.com
Here are a few pictures of the areas in that section:
PCT trail projects can be found at PCTA.org. Click on Volunteer. Come have fun in the mud!