Yes, signed message cannot be abused easily.
This part below is mostly IMHO

Frankly speaking, I believe having only signature way of linking the Bitcoin address would be just enough. This microtracsaction method only messes things up. If user does not know how to sign, he could learn. If his wallet does not have signing capability, he can move his Bitcoins to a wallet which has that capability. This could be a good reason for someone to educate himself, and prove he really wants to have Byteball! Kind of a PoW - "proof of work", "proof of willingness", "proof of worthiness"

This is good point and I agree with that. This will largely simplify the process and prove clearly that you must have the private key of the wallet, avoiding people to send micropayments from some common wallets and take the chance.