Seriously, if you promote the event with a $1 - $5 entry with free raffle ticket for each entry (and also clearly state the money raised is used for the Reef-a-pooloza and realted reef clubs), I don't think anyone will have any issue paying. People pay $1-$2 a person including kids at my local swapmeet just to get in. No one minds a dollar or two, it's really nothing. The hourly give-away sounds like a winner. Would also strongly suggest to keep people interested all day as some people will not be able to come in to later as the driving distance is so far. If the good stuff is gone in the first hour, what's the motivation to drive 1 1/2 hours?
Perhaps have certain selling auctions or events happen on each hour, like $3 a stalk Xenia at 11am, $2 a shroom noon, etc).
If vendors show up, you should have them all donate items to hourly raffles for entry as I beleive they sell out of everything and the trip is well worth the effort for them.
The biggest issue I see is the keeping of live stock alive. If this is the case, you may need to spend most of your time locating a free indoor facility with electricity. Perhaps a Highschool gym? I know our Karate studios have the highschools basketball gyms almost every month end on saturdays for tournaments and testing and demos -- with all the electricity you could want.
Don't forget about the hot chicks at the front desk for $3 entry fee collection. I suggest have eyecandy collect!