Hi Tamas,
We agree! There are great benefits to be had by allowing arbitrary metadata to flow through different parts of the stack. In fact, we enabled a similar thing in Migrations with the anonymousArguments optional parameters on the fluent APIs.
Unfortunately we haven't yet had a chance to enable it for Code First (and I don't think we will get to it for EF6) but I think we would be highly open to a contribution in that area.
Cheers,
Andrew.