Yes, like I said, I was in a rush when I made that post, so my knowledge of how it worked was not completely clear. I've been looking at this morning, however, and I have a better understanding of what is going on.
Indeed, the files are in a compressed archive on the disc, inside main.z. It's not an UNIX archive as TheDoctor gave me a link for, but it is an InstallShield 3.x archive, and can be opened using (Guess what?) UniExtract, namely, i3comp.exe. When opened, most of the game contents come out, such as IMusic, the AVIs, and the zbuffer folder. The rest is the uncompressed data on the disc.
I'm already working on the alternate installer, progress can be found here. I think a first version will be done this afternoon; recreating the Full, Minimal, and Custom options will come later.
EDIT: I meant to ask, are there copies of LEGOLAND that have different layouts than mine? I always want to support as many versions as possible. If your disc is different, will you post an image of your disc root and/or a file tree?
EDIT 2: Also, if your disc as a PDF manual, say so as well, so I can install it if it exists on the disc.
EDIT 3: Installer released, download here.
Edited by le717, 12 August 2013 - 10:28 PM.