How to change ubiquity release name
Have a look on the media itself - there is a folder called .disk. In there is a file called info. The release name is there and can be changed.
source . I would use ISO Master (search for this in Software Center) to alter your ISO.
At a guess, if you use ISO Master to change your master ubuntu.iso .disk/info file, then this should be carried across into your remastered custom distro.
If this doesnt work, then probably just best to use ISO Master post the build to change the release name.