Yes, I'm beginning to think that the "Futures without expiration" or "ETF w/ delivery" idea may be the way to go. It seems like people may use this more to speculate on the price of silver rather than as a way to purchase bullion (which I thought would be the other way around). The great thing about GLBSE is that the listing rates are very reasonable so I could try a few different variations of these contracts and see what works the best.
Yes, I think you'll miss both markets doing it the way you originally proposed. People who want to buy silver don't want to buy it at a specific time far away from now and people who want to speculate don't want to be forced to take delivery or risk having to sell below market to convince someone else to. Continual with redemption is best imo.
I think making shipping fees separate as TT suggested is fine as long as you commit to a fee schedule and a period of notice for changing that.