Maybe I am missing something but the effect as I see it performed can be explained quite naturally. You can also do the effect with a signed coin (provided you sign it!), and the coin stuck in the bottle neck has the same signature.
OK you will claim this is not how you did it! Please prove it and take of the cap of the top of the bottle and
prove to us that the bottleneck is empty in the first place! Then go one to demonstrate how the signed coin penetrates the bottle.
Please do not overclaim! What you show leave three posibilities:
a) Your claim about the signature is correct, but we do not see the full effect you have in mind.
b) Your claim about the signature is correct, but you can improve the presentation on You Tube by showing that nothing is hidden in the bottleneck in advance.
c) You claim about the signature is an overclaim (though strictly speaking all you say is that the coin can be signed, not that it is signed by the spectator!)