> The benefit would be that since the index is just another file in the tar, you can uncompress the format from a plain tar command with no negative consequences besides an extra file being present (the index).
I still don't get how you can extract the index file from the tar.gz without uncompressing the whole gzipped archive first. Plus I believe you cannot extract a file from a tar archive without extracting all the files first (I could be wrong here.)
No, you can extract specific files from a tar archive. I believe it skips through the tar file object headers, which include file length, so it only has to read the headers. That said, there are problems with my proposal, as you pointed out elsewhere here.
I still don't get how you can extract the index file from the tar.gz without uncompressing the whole gzipped archive first. Plus I believe you cannot extract a file from a tar archive without extracting all the files first (I could be wrong here.)