At the Claremont Colleges (outside of LA, includes Pomona, Harvey Mudd, CMC, Scripps, and Pitzer), we've had success with almost the exact same strategy. At our last hackathon, we had a similar turnout (150+) and ratio (>1/3 women). Just to emphasize again the key things that worked for us:

* a commitment to beginners from the start

* a week of 2-hour classes (Hack Week) on building web applications from scratch targeted at non-developers

* a focus on learning rather than winning (although, we did advertise prizes)

* helpful mentors on call through out the tutorials and hackathon

* providing healthy food for those who wanted it

* having a diverse set of organizers, mentors and company reps who participants feel comfortable approaching during and before the hackathon to allay concerns about their participation

In my opinion, every single one of these things makes hackathons a better experience for everyone, so there's no reason not to do them. To say the least, we've had no problem with scaring away the typical "hackathon types" and this year, we're hoping to get close to a 50% ratio.