Jongwook Lee et al. have published "Availability and Accessibility in an Open Access Institutional Repository: A Case Study" in Information Research.
Here's an excerpt:
This study explores the extent to which an institutional repository makes papers available and accessible on the open Web by using 170 journal articles housed in DigiNole Commons, the institutional repository at Florida State University. . . .
Overall, the results confirm the contribution of the institutional repository in making papers available and accessible. The results also reveal some impediments to the success of open access, including impediments linked to contractual arrangements between authors and publishers, impediments linked to policies, practices and technologies governing the repository itself, and the low level of faculty participation in the repository.