I am not sure anything actually needs to be done. Gender equality has been long established by law. Any overt promotion could be demeaning to women. Witness comments like "I don't like to answer the questions related to being a woman at a hackathon when I am being interviewed by media".
The most I would do is advertise to meetup groups and similar that cater to women who are working in technical roles. Anything beyond that, while well-intentioned, could create serious trust problems that the competition is fair. Recall the Salesforce hackathon incident from last year. They went dark on judging and developers did not like that. On related note, Challengepost, which powered this particular hackathon, should have educated Salesforce until they understood that this would happen. Someone wasn't assertive enough.
At AT&T developer summit, there was a prize awarded specifically to the top team that had mostly women. Reaction to that, including that of participating women, was mixed.