I do not know about SIAlertView but normal UIAlertView is shown via separate window. If you want to overlap it you can not do it by changing zpozition, you have to also use a separate window:
// do not forget to keep strong reference for it somewhere!
UIWindow *notificationWindow;
//your frame here!
notificationWindow = [[UIWindow alloc] initWithFrame: some_cgrect];
notificationWindow.backgroundColor = [UIColor clearColor]; // your color if needed
notificationWindow.userInteractionEnabled = NO; // if needed
// IMPORTANT PART!
notificationWindow.windowLevel = UIWindowLevelAlert + 1;
notificationWindow.rootViewController = [UIViewController new];
notificationWindow.hidden = NO; // This is also important!
UPDATE:
To overlap also a status bar use
notificationWindow.windowLevel = UIWindowLevelStatusBar;
to dismiss UIWindow just invalidate strong pointer to it. Something like:
self.strongPointerToYourWindow = nil;