The --export-secret-subkeys command does just that: it replaces the master key with some GNU specific stub packet thing. It's conceivable that they could detect this and reject the uploaded key. In order to avoid that, one might edit the secret key packet manually instead. Just zero fill or randomize all the secret key bits or something. I assume it wouldn't match up with the public key though. Aren't the public and private keys mathematically related? Maybe you can detect that the key is bogus if you try to do cryptographic operations with it. Maybe the operation somehow fails or produces nonsense results. I don't really know enough cryptography to say.