That said, there is never anything wrong with adding just a bit more salt to your hashes.
You're suggesting:ECDSA_SIGN(SHA256(RIPEMD160(SHA256(public_key))+"fixed string"+message))
is more secure than:
ECDSA_SIGN(SHA256("fixed string"+message))
It sure looks more secure! But maybe some super-smart cryptographer will tease out a relationship between all the hashing and signing in the first version and it will turn out to be less secure; maybe she'll figure out a way to factor out the private key if the public key is involved in the salt.
I like the simpler version better.