Hi Guy, greetings from Chile
Right now i'm into a independent initiative to develop a theses repository wich will have imported records from OAI-PMH compliant repositories, and records enetered by the administrators. We are yet exploring the platforms that can help us, so we don't have any proven results yet, but the recipe seems to be like this: use OAI-PMH harvester plugin to import records, and SolrSearch to make the search interface having the harvested data stored on a Solr database. For the records entered by administrators, we are exploring wich plugin is best to import or manually enter data to the Solr database that uses SolrSearch. Also, you can use the OAI-PMH Repository plugin to make your data available to OAI harvesters, but right now that isn't one of our targets.
Anyway, our Omeka examination it's on stand-by, because we are still assesing all the options that we can consider, so i hope that our preliminary insights about Omeka could help you.