Milter-greylist

Hi,

I'm a newbie, so please forgive me if I'm asking a stupid question.
I have already configured milters for smf-spf + opendkim + opendmarc.
I would like to whitelist automatically messages that pass the DMARC tests,
i.e. if there is a header that looks like:
   Authentication-Results: <hostname>; dmarc=pass ...
Anything else should be greylisted.

Is it currently possible to do this with milter-greylist?  If not, do you think
it would be easy to patch milter-greylist to do this?  Or is there a better
approach that I should be considering?

Thanks in advance for any help or pointers to appropriate docs,
Andy