Workaround for pandocs org-parser not handling generic multi-line metadata keys.

org-metadata.py

#!/usr/bin/env python3
"""
Pandocs Org-Parser does not handle generic multi-line metadata keys. Only
their last line ends up in the AST and so this can't be solved (easily) with
lua filters.

We use python to extract org metadata keys before the first non-metadata line
and output them in YAML for use with pandoc's --metadata-file. E.g.

  #+TITLE: Letter
  #+from: Your Name
  #+from: Your Address

  Hello,

should end up as

  title: "Letter"
  from: ["YourName", "Your Address"]

"""
import sys
import re
import yaml

metadata_regex = re.compile("^#\+(.*):(.*)$")
metadata = dict()

for line in sys.stdin.readlines():
    line = line.strip()  # strip leading and tailing whitespace
    if not line:  # skip empty lines
        continue

    if match := re.match(metadata_regex, line):
        key, value = match.groups()
        key = key.lower()
        value = value.strip()
        if key in metadata:
            if isinstance(metadata[key], list):
                metadata[key].append(value)
            else:
                metadata[key] = [metadata[key], value]
        else:
            metadata[key] = value
    else:
        break  # stop at first non-empty, non-metadata line.

yaml.safe_dump(metadata, sys.stdout)