Open access policies such as that of Harvard or MIT are implemented by granting the university some non-exclusive rights on the works written by faculty. This is very useful as it circumvents subsequent copyright transfers (to some extent).
There are a few articles explaining why this is compatible with copyright law, so in common law (see below). I wonder whether similar policies could be designed under authors' rights, in civil law (for instance the french "droit d'auteur"), where rights transfers seem to obey to slightly different rules.
Simon Frankel and Shannon Nestor, Opening the Door: How Faculty Authors Can Implement an Open Access Policy at Their Institutions
Eric Priest, Copyright and the Harvard Open Access Mandate, Northwestern Journal of Technology and Intellectual Property, preprint August 1, 2012