A better system sends the zaps to an escrow for when the user joins. Let them see what's on the table. Otherwise, I could spam you with the intent to send value until you join. This would also encourage personal relays, since the escrow is tied to authentication, and authentication is settled at the relay. Time locked contracts can ensure value doesn't stagnate on uncooperative users. If the claim is not met by a set time, funds are reimbursed to the sender.