Is it really necessary to support backslashes in the file format? The executable that creates the tar archive knows whether the backslash is to be interpreted as a separator or as a filename character. Otherwise it wouldn't be able to add the right files in the archive.
After this interpretation it only needs to normalize all paths to use normal slashes to make extraction deterministic. Of course, if all Windows ports won't implement this proposal, the extracting tar executable is in trouble. Is there any guarantee that the file won't contain mixed slashes and backslashes? - Ville Oikarinen
