Ask HN: How to create a successful Hackathon? 17 years ago
You could try a direct approach if you know who is attending sending them a personal (or seemingly personal) note to tell them you're excited they're joining and what the prizes are. Might also be more specific. You could win $50,000 (not sure how specific you are being). You could win "a new mac pro" versus "a new computer" might entice the developers.