I have done 5 GVBRs and I have never noticed any freeloaders at all.
There are so many riders on the road, I wouldn't worry if a couple of freeloaders joined in.
Just think that, they may ride for one day but if they do more than one day, then the chances of security stopping them entering the campsites are very high. So they would have to camp out overnight or stay in their cars etc... a hell of a lot of trouble to go too.
You can't get any BV food without your current pass, so they couldn't get food for free, they would have to buy it, thus helping the local communities.
I wouldn't lose any sleep over the freeloaders, they have never interrupted my GVBRs. As I said earlier, I have never noticed any.