The original hashcash algorithm was meant to quench email spam which is not completely unrelated.
But in this case I think the parent was proposing tricking the attacker into computing a PoW without realizing it, which sounds pretty tricky.
Just need a protocol that allows for server initiated key renegotiation with PBKDF. Then you trick the client into repeatedly calculating some new key.
You can no longer profit mining Bitcoin with a CPU or GPU. ASICs are needed.
The idea of this sort of scheme is that it's perfectly profitable if you aren't paying for the CPU and the power it uses.