How to tell the "configure apt" part of installer about DVDs that exist only as ".iso" files on a USB stick?

[Rick Thomas <rbthomas@pobox.com>]

Here's the situation...

I want to try installing without internet access.  I have excellent 
internet access at home, but there are enough folks on the users list 
who don't enjoy that privilege.  I'd like to be able to help them with 
useful answers.  Hence my questions...

I've jigdo'ed a full set of the install DVDs.  I can fit the iso's on 
three 16GB USB sticks.  I can boot the machine from a "netinst" CD and 
do most of the install from it.  When the install comes to the part 
where it's configuring apt, I tell it *not* to use any internet mirrors.

At the part where it asks about CDs other than the initial installer 
disk, I'm ready with the USB sticks inserted.  By switching to the 
<alt>-F2 console, the DVD images can be loop mounted from there.

So here's the part I can't figure out...

How do I tell the installer to look at the /dev/loop* devices
instead-of/in-addition-to the physical CDROM drive?


Thanks!

Rick


PS: Wouldn't it be nifty if the installer could do all that without 
switching to the <alt>-F2 console for behind-the-scenes magic?

Once I get the magic worked out, I'll submit a wishlist bug for such a 
feature.