The obvious beta-test candidate for this one would be ECU Silver. Sitting on veritable mountains of silver in Mexico to back said bonds, and at the same time needing to clean up their balance sheet in order to expedite the process of becoming a full-fledged producer. If you haven't approached them with this idea, I recommend you do so.
Silver ETFs Start September with a Bang [View article]
The Silver Indexed Bond [View article]