Custom fields - on the radar at all?
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Essentially you can use Zotero's Extra field for your custom fields, which zotero-better-bibtex will recognize on export ...
Extra: bibtex[origdate=1856;origtitle=An Old Title]
See https://github.com/retorquere/zotero-better-bibtex/wiki/Customized-Exports
(But at this point of time zotero-better-bibtex only supports Zotero 4.0, with Zotero 5.0 support pending).
originals-date: 1784
original-title: An older work
Citation styles supporting these fields will pick this information up.
I'm not 100% sure how that handles unformatted dates, though.