It wouldn't be a CDS, it would just be a swap (like a currency swap). A bitcoin isn't a credit instrument, so there's no possibility of default. (Although it's also true that swaps usually anticipate a currency's local interest rate -- what's that for bitcoins?)

And, you probably wouldn't use a swap for hedging bitcoin currency exchange risk, unless you were doing an actual useful transaction where the sources and uses were in different currencies. People with bitcoins are either speculating or taking duration risk approximately equal to the delivery delay of a Priority Mail envelope full of heroin, as I understand it.