I didn't say it was a good idea to redefine "woman," I said it wouldn't be impossible.
Other legal fictions: that companies have legal personhood. Yet throw a brick through the window of a supermarket and you'll get charged with criminal damage, not assault against the person.
The most similar is probably legal adoption: adoptive parents are parents. "Parents" has a real biological meaning. However in law, and in society, adoptive parents are indeed parents. The courts (Lady Justice Hale in this instance) have interpreted "parents" to include gestational, biological and psychological parents - so a parent doesn't have to have gestated a child or be related; being a very involved step-parent makes you a parent, as does being the gestational mother's lesbian partner and the second mother to the child for example.
I wonder whether in the future we will see a similar tripartite interpretation so that "woman" (and "man") are interpreted as biological (XX or XY), legal (has a GRC) and / or social (passes through life as a member of that sex; the Clapham omnibus test).
That would include trans men (biological but not social). It would include those with a GRC (legal but not biological). It would include someone who lives as a woman and is perceived as a woman (because as liberals point out, the employer who rejects someone because they think the interviewee is a woman capable of getting pregnant doesn't do so because he conducts a chromosome test but because that person looks like a woman and has a date of birth between 40 and 20 years ago). It would not include Danielle Muscato, who the person on the Clapham omnibus would not perceive as a woman.
This is all theoretical btw, I'm not proposing it as a legal test. Quite apart from anything else I've had most of a bottle of wine after an epically shit day 