Evening all,
I know this effect has been discussed to death on here, but I've been playing around with different bottles and coins this evening and I've tried an extra convincer, which worked REALLY well!
Have the spec take a 10p out of their pocket, give them a permanent marker and ask them to put a big cross across the face of the coin, then pass it through face up, and show the marked coin in the sealed bottle!

This is the nearest solution to a signed coin I can think of at the mo!