The application to start is called
gnome-session-properties (part of
gnome-session-bin). There the list can be edited.
Any changes there will then appear in the other list accessible from Settings>Applications>Startup; thus, the latter is limited only to reverting to defaults.
In order to access that in the Panel Launcher (Applications), I have created a .desktop file inspired by the other such files that can be found in
usr/share/applications with this content:
Comment=View apps starting with the system
So that it now can be found when searched or under Applications>System Tools