Fiat by definition is backed by nothing. A gold backed currency is not fiat. And your suggestion that the BRICS nations are going to trust Switzerland, or any other nation, with their country's capital reserve is absurd. The very first thing you need to ensure something like that is a massive military. This is why the US held so much of Europe's gold until they rehypothecated it.
The fear of rehypothecation of gold is far too high for a country to risk it's financial stability on another nation. This is why it's not done.
As far as flying gold around on a plane, that's great unless the plane crashes, is hijacked, terrorized, stolen, lost, and so on.
This is why gold is not used to back any currencies in the modern era. It's way too slow, way too expensive, way to dangerous.
Nations that hold gold, and most do, hold their own. You cannot create a currency which requires trillions of dollars of liquidity and back it with gold. Not since the telegraph has this been a viable solution.