This year, over on the fullsize bronco site, we organized a run to the rocks in Moab, UT. We had about 10 months of notice and planning, and chose to coincide with the Bronco Safari event. Interest peaked at over 60 members, but in the end we had 13 show up, from all over the country: NY, GA, ID, AZ, NE, KS, CA, UT and CO. The bronco Safari event is nearing 10 years old and is growing every year. There were over 100 Broncos of all sizes out there. I expect next year to be even bigger. They have obtained sponsorship from people in the offroad parts manufacturing industry and such. They hold a raffle, and everyone gets something. Top prize this year was a set of 5 US Mags Indy wheels. As well as a set of fuel wheels, roll cage, bumpers, seats, etc...
It's a matter of exposure. The more you put it out there, the more it'll stick in people's minds and eventually maybe they'll want to go. Advertising in magazines can also be good. Bronco Driver is one that comes to mind.
Another consideration is when you hold the event. This time of year, the farmers are in the fields 12 hours a day. Our local ducks unlimited banquet was moved from late October, to last weekend. Attendance was about half of normal, due to this.
Holding it at Grand Lake of the Cherokees might entice more people to come.
Wish I could have made it down there. Next year!