I know the direct function works. It's what I use in the header here: http://dittynewsticker.com/. I've also used it many times on other projects I've done.
It seems like it's an issue with your theme. I have never used the Montezuma theme, but, it sounds like the theme uses it's own implementation of a theme editor that may strip out php or something...
What happens is you just insert the following:
<?php echo 'This is a test to see if any custom php works; ?>
You will probably have to contact the author of the theme to resolve this issue. There isn't anything "special" about the direct function used to call Ditty Ticker. It's just a basic php function that displays the ticker the same way the shortcode displays it. The shortcode actually calls the same function.