As an option, I reached out to Ocotillo Wells Ranger Station to see how much it would be to reserve/rent the amphitheater/event center on Main St. This area has tons of picnic tables, a huge community campfire ring, bathrooms, and showers. It is also located centrally to all of the trails that will be run(I'm assuming).
I know there would be a cost to this, just waiting to hear how much.