I bet you guys love it when people solve their own problems 🙂
The problem was that the Awake theme uses a custom function ( mysite_body_class() ) for setting body classes instead of the standard body_class(), so any classes added by placing filters on body_class were effectively ignored. The fix was to change the following in header.php:
Before:
// header.php
<body class="<?php mysite_body_class(); ?>">
After:
// header.php
<?
ob_start();
mysite_body_class();
$awake_classes = explode( ' ', ob_get_contents() );
ob_end_clean();
?>
<body <?php body_class( $awake_classes ); ?>>