On March 5, the Washington legislature passed SB 6058, which sets up a procedure for write-in candidates to file a write-in declaration of candidacy. The declaration must be filed by 18 days before the election. The bill lets write-in candidates file afterwards if they pay the filing fee for that office.
Unfortunately, the bill does not say that write-in votes for declared write-in candidates must be counted. But assuming this bill is signed into law, it is somewhat likely that a court could order that such votes be counted. The U.S. Supreme Court long ago said the U.S. Constitution protects the right of every voter to have his or her valid vote counted. Thanks to Jim Riley for the news about the bill.