I'd love to see a proof of that. Not meant as a challenge and I don't necessarily disagree at this point. It just seems hard to say that because we don't know of a way there can't be a way, and such a proof would be interesting.

In fact I have a vague notion of idea that may be possible, but I haven't reduced it to a usable form. (Not at all related to PoS or other such techniques, and my idea may too devolve to PoW in some unseen way.)