I've never seen this cited anywhere -- remember that Zotero is principally a reference manager, it can't possible include all important fields for all item types (e.g., DDI, the standard metadata format for data in the social sciences, has several hundred fields, even in it's simple version)

Two things:
While I very much agree that digitized archival sources (or really, any archival sources) constitute data for many researchers, I don't think they are typically cited appropriately as a dataset. If I'm looking at a digitized letter from an archive, say, the right item type to use is still letter.

As for citation contents -- Zotero already offers fields for both physical provenance (archive, loc in archive, etc.) as well as digital provenance (library catalog -- to be interpreted broadly, DOI, URL). Those can be combined to provide pretty good citations for digitized materials that account for their dual character: those citations don't detail more fine-grained provenance, chain of custody, digitizer etc., but I'd say critical assessment of sources is best done where it's traditionally been done in historical work, in discursive footnotes.