Found an open one today! So 1/4 of the local high school tracks is open.
My in-laws are in the San Juan Islands, so I am fairly familiar with your area and tracks. Some I can mention:
Western Washington University (very nice facility and always open when I've been there--in the summer)
Oak Harbor High School (also always open, and they didn't even used to have fences, though you might have to share the track with military guys playing with model airplanes/drones--on the track--and no, I'm not kidding)
Anacortes High School
The purple outdoor track at UW. Dempsey has become prohibitively difficult to get into for unattached runners, but they used to have practice schedule times. I don't know if this is still the case, but there used to be a couple of youth track clubs that practiced there.
There aren't that many tracks in the Bellingham area, and it's probably a 1-2 hour drive for you, but what tracks they have in the area are mostly open, which the exception of Burlington High School.