Whoah… Bit of a long one this. Please bear with me.
I dont think you have to go as far as using chilli water (some dogs like it, some will hate it) etc but I understand where you are coming from, and what you are trying to acheive.
Why not try Noise Association instead..?
Much simpler, less messy, but noisier.
Find a noise that your dog really HATES. A large rattle, or a loud clang (bang a saucepan or something). A horn. You may have to try a few out on him first…
Then, as you were going to, get your friends to chuck the food over the fence. As your dog goes in to get it, just as his mouth opens up – bang that drum!! But only do it once (the short sharp shock treatment).
One loud bang should distract your dog immediately, and make him look at you. Call him over and praise him – give him a treat if you like.
Important that you must only make the noise ONCE, which is why you must make it very loud. If you do it again and again it will just become noise to your dog and he will learn to ignore it.
You will need to practice this over and over and over again. Everyday! For as long as it takes. Praise him every time he gets it right…. give him a right ‘Good Boy!’
Have patience and eventually your dog should learn to avoid going into grab food that’s chucked over the fence (he wont want to hear that noise).
But you should still continue to practice this method so he never ever forgets – although you can probably cut down the practice to maybe once a week, or a fortnight (like a refresher course).
Your second idea however, I dont really get at all.
You seem to be asking him to associate all these things together…
a) chilli water or meat or whatever..
b) how many hands are giving him the food
c) and strangers
Your poor old dog. Try not to set him up for failure (not saying that your method will fail, but it seems too complicated work)
Why not try the noise association method again. Keep it simple and it should work too.
Ask your friends again to try and feed him. Then get on that drum and BANG!
By the way, your friends dont have to be strangers. The point of the exercise is to associate the NOISE with NOT TAKING FOOD… so shouldn’t matter if they are friends or strangers. Basically you are teaching your dog, that the only one who should be giving him food is YOU! You are in charge, your the boss!
Your dog should always be supervised, by yourself or a friend you trust. If you follow these methods be sure to pass them on to the person looking after him. Otherwise they might not know what to do…
But how very sad it is that you have to protect your dog from people who will poison him.
Hope this helps! Good luck!