I'm gathering commands to build a post-installation script, and I want to have the script automagically add certain apps to the Startup Applications list as if I had added them manually, such as the GNOME Login sound, and Xscreensaver. Any ideas?
You don't need to do anything with post-install scripts for this. You simply need to provide the correct .desktop file for your app, following the Freedesktop Autostart Spec. This will enable autostart of the app.