To be honest, a women+ organisation doesn't sound like a womens organisation at all, but a TW focused organisation, where the women are there to support the mens gender identity. The organisation could still exist, but not pretend its for women.
I've been thinking that might be a route you could maybe manage. Say that it's a service, and that you've got a two distinct roles - the customers for the service are the transwomen (males with gender reassignment), and the service providers are women.
That might maybe get you there? There's a genuine occupation requirement in that men with gender reassignment want to be surrounded by women, and that's the service you're providing, therefore all your service providing volunteers have to be women, because that is the specialist service.
Sounds like legal chicanery, but not really - it's just being honest. And that honesty might maybe make it justifiable. You've stated a valid aim.