Well there is a way if you know both the plaintext and the encripted form of a message. But anyway, I'd like to see some kind of proof that bitcoin transactions can be used for that.
Yes, but what you are brute-forcing is set: block and random number to get hash that start with defined number of zeroes. That's it. No other use. No brute-force of communication. It does not even help to brute-force passwords.