You can't dynamically change the plist in the Settings.bundle because it belongs to the read-only (as far as sandboxing goes) app bundle.
What you probably want to do is to modify the current value using [[NSUserDefaults standardUserDefaults] setObject:@"the username" forKey:@"username"]
in your -application:didFinishLaunchingWithOptions:
or somewhere else in your startup process.
Then the change will be reflected in IASK and the system settings.