this is a bit of how Blahaj works as I understand it, so it's a good model - if anything I would think Blahaj might already be poised for this kind of instance-level protection of women

EDIT:

one of the Blahaj guidelines does include removing bigotry, including sexism, and would be a candidate for a safe space for women:

Inclusion and Acceptance

Embracing inclusion and acceptance means listening when people tell you who they are and what their needs are. It means not telling people that you know their experiences better than they do. It means not gatekeeping experiences of identities of others. It means no bigotry such as racism, sexism, anti LGBT commentary, ableism etc. It means doing your best to ensure that you don’t over-talk the voices of folk who don’t share your privileges.

That said, the women spaces on Blahaj are mostly for trans women, so a more general women's community would be nice.