It would have cost them nothing to unveil a hypothetical ~/Uploads directory in the process of patching it to unveil ~/Downloads

You can trivially-easily add it yourself by editing a text file, unveil is configurable per-process.

True, but defaults are worth a million

With what purpose? Why do you want two different directories? On second thought, maybe you mean ~/Uploads could be unveiled read-only, though I still don't believe this brings much.

Yes, configurable in /etc/firefox/unveil.main