Why not BTC? It is like gold, but you don't have to haul pallets of gold back and forth with logistics and security risks.
Remember they don't trust each other, right now, they rather use USD with all the negative aspects over their own currencies.
I'm definitely not saying they will promote use of Bitcoin by their own people. Quite the opposite.
BTW wasn't this already the model during the cold war? Eastern Block would be doing international trade in Dollars, but their own people would be banned from owning any foreign currency.