Recently, we released the ArchivesSpace Connector module for Omeka S. The module uses the ArchiveSpace API and a collection identifier to import Archival Objects as Omeka Items. When used with the Hierarchy module, the module can also maintain the ArchivesSpace collection structure by importing collections/series/subseries as hierarchically organized Item Sets.
Developed by Matthew McKinley, this module was the result of collaboration and consultation with users in the wider Omeka S community. Many thanks to Arden Kirland, and our friends at the University of Missouri - Kansas City for discussing their use cases and requirements!