Sounds awesome, I'd love to attend something like this. For me it doesn't matter if it's in the middle of nowhere or at an improved site.
Location and time of year are connected due to weather factors.
St George is awesome! There have been some very successful winter 4x4 events there in the past or early spring / late fall is also very pleasant.
Someone mentioned Green River. We were looking into hosting an event there and it has 2 nice private campgrounds and several hotel options (Super 8 down to some $30/night dives). Also the city was VERY willing to help, provide support and provide a city park for the event. They'd love to have tourist traffic. It's a great jumping off point for interesting day-trips in every direction, it's even close enough to Moab to make a day run down there. The Swell, Goblin Valley, Sego, Thompson Springs, Crystal Geyser, or even a long day trip to 9 mile canyon. Best time would be late spring or early fall for that area.
Southern Colorado would be great. Grand Junction or Delta area? Grand Mesa? Pretty good through the summer?
Flagstaff Arizona is a cool area.
Wyoming? Pinedale or northwest WY?
Death Valley in the winter?
Do something different and go end to end instead of static? Mojave Road? Pony Express through Nevada and/or Utah? This might limit the attendance, but an all-70 series multi-day trip would be rad.