dpkg-mountable adds an access method called `mountable' to dselect's list,
which allows you to install from any file system specified in /etc/fstab. For
example, the archive could be a normal hard disk partition or an NFS server,
which it will automatically mount and umount for you if necessary.
It also has some extra features not found in the standard dselect methods, such
as provision for a local file tree (either parallel to the main distribution or
totally separate), and only getting packages which are required, rather than
the time-consuming recursive directory scan, as well as logging of all dpkg
actions in the install method.