I have done a coin routine in which I discovered a spectator's signature from a form (I made sure it was fairly recent) and copied it carefully onto a bent coin. I then got them to sign a coin and then a typical switch for the preprepared one. They didn't match very well from looking at the spectator's signature, but the spectator recognised it as their signature. I don't use it often and have only once tried that signature thing above so I don't know how practical it is if you were going to use it regularly.
I was just being an opportunist because I practically had their signature handed to me on a plate ( I keep getting given signed plates.. don't ask) but I suppose you could do some preshow work and get them to sign something to get their signature like an ambitious card routine before the show. Just an idea.
I suppose that if there was a clear problem with how they signed it during the performance then you could go ahead and cleanly pretend to bend it with your mind. Of course it fails and you hand it back to them with an excuse about why it might not have worked- not only is that an opportunity to show them how the real effect would look and let that visual memory become confused with what you are about to show them it also suggests the coins aren't switched (because of the signing) and gives you the opportunity for a remarkable effect. You'd then start again with a fresh unsigned coin Then You would switch it for a second bent coin without a signing in it and then switch it.
Or you could just ditch all that and buy a gimmick. I'm cheap though

''To go wrong in one's own way is better than to go right in another's.'' Dostoevsky's Razumihin.