Sounds cool. I'm not sure of tar's file format. I'm assuming that it's something like

[
  [ <tar file header>         ],
  [ 'path','file','perm','data'],
  [ <another file>             ],
  [ <yet another>             ],
  [ <tar file done>            ]
]
[download]

Because tar ztvf file.tar.gz takes too long for there to be a full directory at the beginning of the tarfile. So I'm thinking, enforce the manifest to be the first file in the archive. I wouldn't want to have to uncompress a huge archive because the manifest was the last file and not the first.

I'm pretty sure tar can do this, don't know about plugin creators ;)

Or I could be wrong about tar... Just a thought.