Not wanting to wait for a response from the moderators, I've found one solution for this.
This is based on a joomla.org forum post:
forum.joomla.org/viewtopic.php?f=431&p=1990059
in the layout.php for your theme, modify this line:
<span><?php echo $item->title;?></span>
Replace with this:
<?php $split = explode('||', $item->title, 2);
$title= $split[0]."<em>".$split[1]."</em>"; ?>
<span><?php echo $title; ?></span>
Then, in the menu item title, just add a double pipe ('||') between the title and subtitle.
Obviously, you'd need to style for that contained <em> within the stylesheet.
Hey, just had a quick question about this. Where exactly is this 'layout.php' file? Thanks a lot!