also, and i won't say who, but someone i recently spoke to proposed another idea; although not yet tested in courts to either of our knowledge, article 16 of the GDPR might be possible to use, to force courts in european countries to provide x marker on passports (or leave the gender field blank).
i'm actually eligible for irish passport so i might consider getting an X passport :S (ireland allows X marker)
then i can use my british F passport when I feel like it, and irish X when appropriate.