Most Product Camps have been very successful with "dot voting" where everyone gets some stickers ("dots") to stick onto the talks/sessions they like the most.  Organizers then just count up the dots to see which sessions win.
With 500+ likely attendees, we want to avoid a mob scene so early in the morning.  We want to get the MORNING agenda picked a few days in advance, and still have "dot voting" for the afternoon as everyone arrives.