I thought about the ISO setup a while ago when I wanted access to local Mandrake RPM repositories (I wasn't on broadband then) and I wanted to maintain the ISO files of the distro (and contribs, and PLF) so I could burn them for friends.
Basically what I did with Mandrake was I kept the ISO files on the harddrive, created separate mount points for each one (/mnt/install-cds/disc-X), added entries for each file into the fstab, ensuring that I give it an option "loop":
/mnt/files/isos/mandrake-9.1-disc-1.iso /mnt/install-cds/disc-1 iso9660 noauto,users,loop 0 0
Then I would mount each directory and use Mandrake's package manager thingy to define some new RPM repository, ensuring that I selected "removable media" for each. This way whenever I needed to install something from the "CDs", it'd loop-mount the ISO file in the appropriate place, pull the RPMs from it and then umount it again.
Now, I know we're talking about Debian systems here, but the principle should work in the same way. Create/keep the ISO files of your 14CDs on your harddrive somewhere accessible, create mountpoints for each one, create separate entries for each one in your fstab, edit your sources.list accordingly.
If you have no need to keep full ISO images on your drive, you just want to keep the debs, then I can see no reason why you can't just copy them (keeping the directory structure in tact, perhaps) somewhere and then have your sources.list file point there.