3SAT with an oracle for expanding the clauses
Your question seems closely related to the question of whether $NP=coNP$. It seems that the existence of such an oracle would imply $NP=coNP$ since the oracle would give you a polynomial certificate for "no"-instances of $SAT$.

I don't think this would be a viable way to show $P\not = NP$ since all you'd be showing is that there's no way to expand clauses in polynomial time, but that does not rule out other approaches for solving $SAT$.