If you dont have direct control of the supply, you wont be able to control it for long. You will eventually run out of funds, there are very clever speculators out there. This happens constantly when central banks try control the exchange rate of their currency. Because they dont control the supply of the other currency they get crashed. The chinese are able to do what they do because their economy is developing and the natural tendency of their currency is appreciation. Devaluating is easy and "free" (for the central bank). Increasing the value of a currency is expensive. If the chinese economy started to fail, their central bank would be unable to hold the value of the yuan to the dollar.
With a 100% backed currency, you can hold the value to almost anything.
In this example, if the price of the currency rises above (beer price) + 3%, then he could buy lots of cans of beer. He could then sell the beer if the price of the currency falls.
The fact that beer doesn't last that long would mean that he would have to cycle the cans out of his reserve.
Storage costs in general would add more expense. OTOH, if he bought in bulk, he could very easily buy the cans at lower than the high street price.
All he needs is for his reserves to be worth at least as much as the outstanding coins. Worst case, everyone turns in their coins and he ends up with whatever is left in his reserve.
If people paid $2 for a coin and he can buy cans for $1.50, then he can build up his reserve at less than the face cost.