How do you get template preprocess functions to fire on hook theme suggestions?
You need to do this before you can do that...

/*
 * Implements hook_preprocess_field().
 */
function YOURTHEME_preprocess_field(&amp;$vars) {
    if ($vars['SOME_FIELD']) {
        $function = 'YOURTHEME_preprocess_field__'.$vars['SOME_FIELD'];
        if (function_exists($function)) {
            $function($vars);
        }
    }
}