I'm not sure whether it would work in general, but at a university hackathon, making the event a way to 'prove yourself to recruiters' helped us get a good turn

I'm not sure whether it would work in general, but at a university hackathon, making the event a way to 'prove yourself to recruiters' helped us get a good turn-out. We didn't collect resumes during the hackathon (and in general, we didn't permit any 'company pitches'), but about a week or two afterwards we sent out an email saying "if you'd be interested in working at any of our sponsors (http://pennapps.com/our-sponsors/ http://pennapps.com/our-sponsors/), send us your resume and we'll forward it on." This lead to (both times we've done this) about 50% resume turnout.