Short answer: use forceFetchGit = true; and update the hash. IIUC the contents will have the raw file in it. You can script a suitable placeholder replacement in postPatch.
Long overengineered answer: You could also get a tarball without the git archive postprocessing by producing a URL for a tree hash instead of a commit hash. You can find the tree hash using the GitHub API or the git client.
This fetchTree issue has more details, although in the context of non-FOD fetching.