This is not possible in Simple XML. Since the result is an html page however, you can do whatever you want with it. For example, you could split your title into two DOM elements and style those accordingly. Given the following Simple XML:
<title>Panel Title, many big words</title>
The following jquery places the word "big" in a new span with an additional class:
@sarit_s - Did the answer provided by jeffland help provide a working solution to your question? If yes, please don't forget to resolve this post by clicking "Accept". If no, please leave a comment with more feedback. Thanks!