Mount a tar file - not possible?

Traditionally, no it's not possible.

However, you can do so using the FUSE filesystem archivemount under Linux.

IMNSHO this is a bit of a novelty and not usually necessary, but I can see a few edge cases for it.

The package is available in Ubuntu 11.10, alas broken.

One can install it via this workaround:

mkdir -p ~/src
cd ~/src

sudo aptitude install build-essential devscripts debian-archive-keyring libfuse-dev libarchive-dev
apt-get source archivemount
gpg --keyring /usr/share/keyrings/debian-archive-keyring.gpg --verify *dsc

cd archivemount-0.6.1
cat << EOF | patch -p0
--- debian/control
+++ debian/control
@@ -8,7 +8,7 @@

 Package: archivemount
 Architecture: linux-any
-Depends: \${shlibs:Depends}, \${misc:Depends}, fuse (>=2.8.5-2)
+Depends: \${shlibs:Depends}, \${misc:Depends}, fuse-utils (>=2.8.4-1.4ubuntu1)
 Description: mounts an archive for access as a file system
  archivemount is a FUSE based file system for Unix variants, including Linux.
  Its purpose is to mount archives to a mount point where it can be read from
EOF
dch --nmu 'Depend on fuse-utils instead of fuse on Oneiric Ocelot.'
dpkg-buildpackage

cd ..
sudo dpkg -i archivemount_0.6.1-2.1_amd64.deb

The last step did not work for me, so I

cd archivemount-0.6.1./
configure && make && sudo make install

instead.