Just to play devil's advocate:  you're right that the customer would not have to trust you, but someone receiving a loaded coin from somebody other than you would instead have to trust whoever loaded the key - someone they will never be able to identify let alone take action against in the event of a fraud.

A user could already remove the counter-party risk on a Casascius coin, simply by printing his own private key on a round piece of paper like I do, attaching it to the coin under a round sticker of his own choosing, and funding it himself.  But by doing so, his coin would not have the property of being spendable to somebody else who doesn't trust him, because that next party has no independent way to know who generated the key within it and/or whether they still have a copy of the key.

Using NFC or RFID or any other similar technology in place of the paper doesn't improve on that problem in any useful way.