That's an interesting thought. The reason why the wallet needs to be unlocked is to be able to access the address private key in order to sign the message which is then sent out to the network. If the vin, address and masternode private key remain the same, there should be no reason to have to unlock the wallet again in the future to start the same masternode at a later time. One could think about reusing the same communication. Looking at the current implementation, there is also a check for the protocol version, but this could be excluded from the signed message I guess.