A filter to store image captions into the Meta fields

figure_caption_filter.lua

function RawBlock (raw)
  local caption = raw.text:match("\\caption%{[^%}]+%}")
  if caption then
    captions:insert(caption)
  end
end

function Pandoc (doc)
  -- append collected captions at the end
  doc.meta.figures = captions -- This works but latex is escaped by \textbackslash
  -- doc.meta.figures = {pandoc.RawBlock("latex", captions)} -- This produces error
  return doc
end