Not bitcoin but forcing a proof-of-work scheme has been proposed as an anti-DoS measure before: http://www.csc.kth.se/utbildning/kth/kurser/DD143X/dkand12/G...

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.