If you have to work on a Demoshield installation, keep in mind that the Demoshield part is not the actual install. It is a wrapper / player that is created, usually for a CD autorun, to provide a menu interface to one or more installers.
The actual installer may be of any number of install types including an InstallShield EXE, or a regular MSI.
If you have to script an install or deployment you can probably skip the Demoshield part altogether and just run the installer itself.
Demoshield is a discontinued product of MacroVision, the same people who produce InstallShield.