The startup script support was for Sytem Startup, i.e., booting, but can and will be extended for dynamic extension processing (tce-load). However, they are not an autorun for extensions for the following reasons...
There is a big difference. During booting, no login has occured. No X started. No paths sets. During booting it is like an rc level script. With dynamic loading (tce-load), X is typically up and user is loggged in. Paths are set. Therefore, startup scripts are not an autorun for extensions.