metadata about relationships

In general, schema.org (or microdata in general?) seems to lack the ability
to specify metadata for the relationship between items.  I'm curious if
there is a general pattern for how this data should be modeled.

First a concrete example.  The Person type describes an individual person,
and the EducationalOrganization type describes an actual education
institution.  The 'alumniOf' property defines the existence of a
relationship between a Person and an EducationalOrganization, but there is
nowhere to provide additional metadata about that relationship.  Things
like graduation year, field of study, etc.  Similarly for work information
defined by either 'worksFor' or the more general 'affiliation'.  I know
there has been discussion of including a CV schema which would likely
address these specific examples, but this seems to be a more general
problem with RDF style triples.  There's plenty of space to exhaustively
define the subject and object, but no room to provide metadata around the
predicate.  How is this type of thing handled in general?

And more specifically, how should it be handled in the above examples?  We
include this metadata in Google+ profiles, and are able to include it
cleanly in Portable Contacts, but I don't see how to represent it in
schema.org.

-will