Sadly, no. You can still initiate a request to login, and not complete it, to consume server resources. Repeat as fast as needed to consume all resources on the server.
How about:
Distributing a tx with a fee and your ip address (in OP_RETURN). Then the server whitelists this ip for x hours? Could this work?
If done for a very short time, or for a limited number of requests, then yes. But it would also complicate the system and you'd have to pay and store data in the blockchain.
