Hmm...not a problem that's easy (or maybe even possible) to overcome in a quiet environment, I'm afraid.
It's for changes like this that the Raven and Black Widow were invented...but then you're just adding yet another utility into the mix.
That said, I'm pretty certain this is going to be your only realistic option; physically sliding a shell across a coin WILL lead to them talking, and I really don't think there's any solution to that
Alternatively, you could look at something like a Coin Unique, which is just as gaffed as a standard shell, but would provide a more visual vanish of a coin (no transformation though, I'm afraid), and is somewhat examinable too
