I can't speak for Mailscanner, but I know MIMEDefang has some built-in RBL functions, and people have posted several greylist implementations that work within a MIMEDefang filter. With that setup, you could have MD do the RBL lookup and conditionally run the greylist code.
Yeah, I could do that. Mimedefang would effectively be acting as the "milter that wraps another milter" in this case.
MailScanner definitely could not do this. MailScanner runs post-delivery, so at that point, any greylisting is impossible.
