Santa Margarita Lake would be the way to go. I've fished Lopez, too, but it has poor launching facilities and you have to contend with jet skis and water skiers.
At Margarita they have two launch ramps, the main one at the marina and a second one to the east. I've seen them close off the east ramp for launching sailboats for an organized event, maybe they can do the same for us? If not, there is still plenty of good shore spots for launch, weigh-in, etc. Good picnic grounds and shouldn't be too much fishing traffic as long as we pick a weekend when there is not another tounament going on. Plenty of Central Coast float tubers in the area.
Let me know if I can help you organize this.
Ken A