One 'interesting' aspect of an interop test for greylisting is that it would
only affect servers. Greylisting is supposed to be backward compatible with
standards-conforming clients. What you would want to establish via
interoperability testing is that a server that implements greylisting per the
RFC doesn't degrade interoperability with existing, properly-implemented,
clients.
I don't have a strong opinion of BCP vs. standards-track for this. I think
that if it's desirable to publish an RFC on this topic (which I doubt), either
BCP or PS would suffice. But I do think that if it's going to be published as
PS, the document needs to define testing criteria for advancement.
Keith