I have a template which is included in PAGE_CONTAINER, thus present on every page. I want to add a variable to this specific "global" template. I tried the callback tag, but callbacks only support strings as return value. Of course it's possible to generate the string which would be used as-is in the template (in this specific case a select dropdown menu). But that's unfortunate because I want to separate view from controller. Is there a better way to achieve this?