Following discussions on the mailing list and taking into account general evolution of the schema.org vocabulary over recent months, I have produced an updated version of the straw man initial proposal in the Wiki.
This version is somewhat simplified when compared with its predecessor. The objective being to approach a minimum viable proposal for sharing information about archives and their resources to aid discovery on the Web.
It includes a worked through examples of an item in an archive, showing the overall data model plus examples of data in JSON-LD form that could be inserted into the individual html pages for the Archive, ArchiveCollection, and ArchiveItem.
The prototype working example site for Schema.org including the proposed extension has been updated to reflect these changes: archive.sdo-archive.appspot.com.
I look forward to continued discussion, moving us forward towards drafting an agreed proposal to the main Schema.org community.