I have a friend who lives in that area, we used to go to the auto flea market that is held on the grounds. He charged people to park on his property during the boots and hearts fest, and told them all the money was going to a fund residents had set up to block the site from expanding. He warned that parking anywhere not permitted (signed as festival parking) could get you towed and towing cost are not cheap, especially during the festival, and that not all the towing companys hired those weekends/events are local, so your car could end up a lot further (like a different town) than the designated parking. The city by-law folks are also cracking down on residents who rent out parking on their property (it's against residential by-laws), so even though you paid some one you could still get towed (though not confirmed). The festival submits transit plans ( approved routes, parking etc) to the city. The routes approved are strictly enforced.
There was only one way in and out (as you mentioned). If you are attending and take a route not set out to avoid the traffic and are stopped by OPP or festival workers you'll get turned back to 11 highway. Which can make a slow go even more frustrating since there is usually a slow crawl on those routes as well, making you're journey even longer.. His advice was just stick to the routes/parking set out by city & festival organizers, otherwise you'll be in for a longer and bigger headache. My buddy says its bad enough during the festvals that he parks his car at friends place a couple kms away and travels by bicycle to get to it. But since its not a full weekend deal it may not be as bad as the above sounds.