I completely understand your concerns, Datun, and agree with not giving out details of the venue itself.
I know that in my case, while I'd love to meet other Mumsnet feminists, the primary reason for wanting a meetup beforehand is so that we can go into the meeting together. It's sensible to assume that transactivists will somehow get knowledge of the venue details and there will be a protestor presence outside. Some of us are a bit apprehensive about walking past them alone - it'd feel much safer in a group. So meeting up inside the venue wouldn't really help there.
Perhaps it'd be possible to arrange a meeting in a central London pub before the venue is announced - that way we're not giving anyone any clues that would compromise the integrity of the meeting, and the venue itself should be easily reachable by tube or bus from a central location.