There MIGHT be riskes with signing public/private keys, but I don't know enough about ECDSA to prove it false or true.
"We" thought of that (where "we" was actually Khalahan and Pieter and Gregory, if I recall correctly).The string "Bitcoin Signed Message:\n" is prepended to the message as salt, then that's SHA256-hashed twice, and the hash is what is signed/verified. Sign anything you like, it won't reveal information about your keys (unless your OpenSLL implementation has a broken random number generator and doesn't generate unique signing nonces).