Reporting back with a solution that works with a test instance that I have not tried on prod.
This is for future me to pass on to the team who will actually be doing the migration or anyone else who needs something similar.
Attachments should be given the long form name, which can be converted from the short form in the json via the python script in Converting short upload URLs to full URLs
Attachments should be placed in discourse/public/uploads/tombstone/default/original/
Then run uploads:recover_from_tombstone while in /var/discourse/