Developing an Element Set Plugin

Our department is thinking we might want to develop a plugin for using a DACS element set for our archival collections in the future. I want to make sure I understand how the plugin would function though. If we were to create such a plugin would it be running parallel to Dublin Core or would it replace Dublin Core? If parallel, would the DACS elements be invisible to OAI-PMH harvesting?

would it be running parallel to Dublin Core or would it replace Dublin Core?

Running parallel. Dublin Core is an indelible part of Omeka.

would the DACS elements be invisible to OAI-PMH harvesting?

By default, yes. But you could map DACS to Dublin Core by hand or automatically using the after_save_item hook. You could also extend the OaiPmhRepository plugin to include DACS.