First of all, just because it's difficult to prove theft doesn't mean it's not property.  Also, you're getting into difficulty of proof, which is irrelevant to what the law is.  

Second, how can you argue that you don't have property rights in BitCoin and then turn around and say you could recover for them in a civil action?  You're contradicting yourself.  You can't recover a judgment for theft of something you never had any rights to in the first place.