SiteModuleControl is a base class for custom modules that plug into the content system, it is not something designed for embedding directly in layout.master. Modules that plugin to the content system have a moduleid and pageid associated with them when they are added to a page through the content system and they do not appear on non cms pages such as the login page.
You can embed a custom UserControl in layout.master but if your goal is to put it directly in layout.master it probably should not inherit from SiteModuleControl which is designed for content system plugins.
Hope that helps,
Joe