If you are not able to prove ownership of a bitcoin address .... you're not satoshi 
Proving ownership of an address proves... ownership of an address. Nothing more. Whoever signs Satoshi's address could have obtained the private keys from him.
Ultimately the only power Satoshi has left is his however many coins so it would be relevant. Beyond that he's been left far behind and that's what he wanted all along.
It prove only ownership of an address, yes of course, but eventually this will be one of the best proofs to show.
Imagine if there are more "satoshi" addresses able to sign a message in the same time...
Personally I don't think such scenario could be classified as a simple "proof of ownership" of these addresses.