We did the same thing in Fort Collins, CO earlier this year. It was a local Church that does a show each year so the attendance was not huge, probably 100 or so show cars, but for a partial one day event it was quit good. We had 7 racers I believe and ran 4 races with probably 20-30 people watching each race. It was a blast and look forward to doing it again next year.
I've always thought car shows or even county fairs would be good places to show RC racing from a spectator perspective and to try and grow the hobby. Unfortunately, it takes time to set these up and I know from my work schedule it's tough.