Marking for cookie isolation makes sense, but could be done more effectively via standardized metadata sent by the first party themselves rather than a centralized list maintained by a third party.

Informing decisions about blocking doesn't make much sense (IMO) because it's little more than a speed bump for an attacker. Certainly every little bit can potentially help but it also introduces a new central authority, presents an additional hurdle for legitimate operators, introduces a number of new failure modes, and in this case seems relatively trivial for a determined attacker to overcome.