I just joined this group, and it looks like I'm exploring ground already plowed (or is that mixing metaphors).
"I’m looking to help improve the story for Linked Open Data in PDF, to lift PDF (and other formats) from one-star to five, perhaps using XMP. "
looks like there was some discussion a few years ago, but not much recently. Were there experiments? Difficulties? Using XMP for document metadata and then either embedding or linking to a PDF/A-3 attachment for each data set, for example?