Setting unix permissions in a ZipFile in a portable python script

Question

I'm trying to create a python script which places a number of files in a "staging" directory tree, and then uses ZipFile to a create a .zip archive of them. This will later be copied to a linux machine, which will extract the files and use them. The staging directory contains a mix of text and binary data files. The section doing the writing is in this "try" block:

try:
    import zipfile
    zipf = zipfile.ZipFile(out_file, 'w', zipfile.ZIP_DEFLATED)
    for root, dirs, files in os.walk(staging_dir):
        for d in dirs:
            # Write directories so even empty directories are copied:
            arcname = os.path.relpath(os.path.join(root, d), staging_dir)
            zipf.write(os.path.join(root, d), arcname)
        for f in files:
            arcname = os.path.relpath(os.path.join(root, f), staging_dir)
            zipf.write(os.path.join(root, f), arcname)

This works on a linux machine running python 2.7 (my main goal) or 3.x (secondary goal). It can also run on a Windows machine (sort of an afterthought, it might be useful), but there's a problem with permissions in that case. Normally the script sets permissions in the files in the staging_dir with "os.chmod", and then zip creates the archive with the right permissions. But running this on windows, the "os.chmod" command doesn't really set all linux file modes (not possible), so the zipfile contents aren't at the right permissions. I'm trying to figure out if there's a way to fix the permissions when making the zipfile in the code above. In particular, files in staging_dir/bin need to have "0o750" permissions.

I've seen the answer to How do I set permissions (attributes) on a file in a ZIP file using Python's zipfile module, so I see how you could set permissions with "external_attr", and then write a file with "ZipFile.writestr". But the "external_attr" doesn't seem to apply to "ZipFile.write", only "ZipFile.writestr". And I'd like to do this on a zip archive that contains some binary files. Is there any other option than "writestr"? Is it be possible to use "writestr" on large binary files?