By signing a message with the private key corresponding to the public key of one of your output transactions? I think someone who accepts that as proof doesn't

By signing a message with the private key corresponding to the public key of one of your output transactions? I think someone who accepts that as proof doesn't understand what is happening. I could sign "I totes have 9.99 BTC" with the private key corresponding to the 9.99 BTC change output, and if you were a dab hand with the OpenSSL command line you could verify that the signature matched the public key, but you cannot verify that I have not already broadcast a separate transaction moving those 9.99 coins to another address (either mine or that of another party). At that point I might as well give you a signed message saying "I totes am capable of signing messages. This one is signed with a random number. Go me!" Did you have a different strategy for demonstrating that I control the Bitcoins that aren't fixed with 6+ confirmations yet?