HI

if you are replacing the default Controller for all doctypes and templates using

DefaultRenderMvcControllerResolver.Current.SetDefaultControllerType(typeof(MySuperGlobalController));

then your replacement controller should inherit from umbraco's RenderMvcController class.

public class MySuperGlobalController : Umbraco.Web.Mvc.RenderMvcController

if you do this, then calling base.Index(Model) from within your Index method will fall through to the controller finding logic in Umbraco and any other controllers (such as your doctype specific ones) will be fired too after your catch all one has ran.

so something like this ?

public class MySuperGlobalController : Umbraco.Web.Mvc.RenderMvcController 
{
    public override ActionResult Index(RenderModel model)
    {
      // do the global stuff you need to here.
        return base.Index(model);
    }
}