Be bold, turn the signed coin over and let your friend push in to the machine. or rip the bag open just a lilbit and ditch the coin, then let your friend open the rest and find the coin inside.
Perhaps using a Shell and let the signature be signed on the shell, that way u can show the entire signature just before putting the coin in to the machine.
Actually some idea just popped in to my head, how about letting him sign a pre-bend coin without him knowing, it would be totally weird to have a coin like that with signature apear in the bag of chips.