Why would you need to repack a tarball just because it contains
prebuilt docs (non-DFSG-free licensed documentation aside)? I'm all
for rebuilding those at deb build time just to be sure you have the
right deps packaged too, but if the ones in the tarball are built
from DFSG-compliant upstream source, included in the archive for
that matter, then leaving the tarball pristine shouldn't be a policy
violation, right? That's like repacking a tarball for an
autotools-using project because upstream is shipping a configure
script built from an included configure.in file.