No, I can tell you. The constitutional system of electoral votes is not changed by this. Even under the compact, each state is picking it's prescribed set of electors and they are voting for the President. All that's changed is the method by which the electors are chosen. And there, the constitution gives the state legislatures carte blanche. Each state may pick its electors by whatever method each decides. They don't have to even hold an election and could instead simply pass a resolution giving the state's electors to a candidate.
Since this scheme only changes how states pick their electors, not what the electors do or how the system works, there's nothing unconstitutional about it.
This scheme subverts the original rationale behind the electoral college; prevent the populous states from dominating the smaller ones. That rationale is well known and was made explicit in the Federalist Papers, IIRC. There are a lot of other aspects of the Constitution were the underlying rationale for one provision or the other is not explicitly stated. This is because the Constitution implements decisions already argued over and resolved in the proceedings of the Constitutional Convetion. Could or, rather, will such an argument support a challenge to this agreement before the Supreme Court?