As to it not making sense to back a currency with a currency, well, maybe it doesn't need to make sense. What is a 'reserve currency' but backing in one currency for another?
...Come to think of it, United Kingdom Britcoin (UKB) might for traditional reasons like to be "backed" by silver, How much silver would you sell for one UKB?
-MarkM-
OK, I realize we've all entered bitcoin for various reasons, but backing a new bitcoin-like currency with a fiat curreny that's already losing its status as such is somewhat bizarre. The US$ is a reserve currency because, in 1971, Nixon and Kissinger succeeding in getting everyone trading in oil to convert their local currency into dollars to trade (buying and/or selling). Basically, they agreed to develop Arabian oil fields if all oil trades were done in dollars. The dollar therefore went from being backed by gold up until 1971 (for international trade, anyway) to pretty much being backed by oil... as long as everyone went along with the currency conversion (Sadam Hussein decided against this oil for dollars trade, btw). This is why other countries have reserves of dollars.
However, with China and Russia having agreed earlier this year to side-step this $ for oil conversation and trade directly with each other withouth involving the US$, and with the Fed having to buy more FRNs as other countries aren't biting like they used to, the petrodollar is pretty much doomed. It's therefore hardly a wise decision to back a new digital currency with a failing fiat one, is it?
As to your last question. I wouldn't sell silver for a UKB, or at least would hope I wouldn't have to. If silver, or any precious metal, is backing a currency, then you'd have to be in dire straits to decide to liquidate your valuable underlying asset for said currency. So, theoretically, I'd happily use a UKB to buy gold and silver all day everday, but not the other way around, unless I really had to.