Please note that customizing Ubuntu for Wubi may require some additional steps, compared to customizing a standard LiveCD.
The Wubi Customization section in the official Wubi Guide contains some hints and tips.
The guide also talks about customizing Wubi itself and using Wubi for non-Ubuntu distributions, so I have included the points I think are most relevant for your needs/problem, along with my comments:
A Live CD with ubiquity + casper + lupin-support (which provides lupin-casper)
- If your custom LiveCD boots and works, then you already have ubiquity+casper; just check that you don't remove lupin-support during customization.
The ISO must have a .disk/info formatted like the one in the Ubuntu ISO and with data matching what you provided in the isolist.ini
- This is very important - a customized CD is always going to contain more/less/different files than a stock Ubuntu LiveCD, so you will need to ensure that the .disk/info files contain this list, and also modify
isolist.ini with this information.
If you want users to be able to download the ISO, in your web server you need to provide a metalink file for each ISO provided, the md5 of said metalink files, and the signature of the md5s. To verify the signature you need to add the signing key to data/trustedkeys.gpg. If this is too much trouble, you can disable signature checks in data/settings.nsh. The metalink file must be on a "stable" URL since that is hardcoded in isolist.ini. The metalink file can in turn point to all your mirrors. You will notice that 2 metalink files are provided in isolist.ini, one is for final release, and one for development versions.
- I don't think this matters if you are using the local ISO for installation.
Your Linux distribution must have a mechanism to do unattended installation using a preseed file. Wubi takes advantages of debian-installer/ubiquity presiding. If you use a different mechanism, you will have to modify src/wubi/installer_preseed.nsh. The make prerequisite scripts may need to be updated as well, since they are Debian specific.
- Preseeding refers to the automated/unattended installation script, which basically contains answers to all the questions users are normally asked during an interactive installation. Your customization may modify some of these questions, so you would have to modify the preseed file accordingly.