loopback fs as an apt source?

[twb <fubarbaz@bigpond.com>]

(Just passing throgh, so please CC: fubarbaz@bigpond.com)

As a matter of course, I keep the Woody ISO9660 images on my hard
disk.  Back when I set up this machine, I couldn't work out how to add
respositories on loopback filesystems to apt, so in order to use them
I had to `extract' the images to the hard disk.

So basically what I want is for APT, using /etc/fstab, to
transparently (un)mount the loopback images, so I can work directly
with .iso images.

(I can't mount them all at once (at boot), because I run out of lo devices.)

Is there a way to trick e.g. apt-cdrom into doing this?

-trent

ps: do file:// uri's get cached in /var/lib/apt/cache?