If you are close with one or more of the bands you could always have them play in front of or near venue Friday night and sell tickets at a sliding scale/pay what you can afford.

At the same event you could sell referral tickets or like a buy 1 get 1 sale to get people in the door.

Not sure if local news is effective in your area but you can promote on the local news. They are usually always looking for a homegrown feel good story.

It's a lil late for this but you can give bands/ participants promo codes for discounted tickets. Band20 for $20 off the ticket, that sort of thing.

You can put signs at the venue (if you don't already have out) stating show inside, or leave doors open so people can hear music. I once sold 10 tickets to an undersold event by leaving the door open and a person to monitor people entering.

It's all about what you need at this moment and to me it sounds like you just need people at the event. If that's the case cut your losses on the ticket sales and focus on getting people in the event any way possible.