<p>Is it possible to have XML-embedded JavaScript executed to assist in client-side (browser-based) XSL transformations?  How is it done and how official is it?</p>

<p>Microsoft's XML DOM objects allow this on the server-side (i.e. in ASP/ASP.NET).</p>

<p><strong>Clarification:</strong> I do not mean HTML DOM scripting performed <em>after</em> the document is transformed, nor do I mean XSL transformations <em>initiated</em> by JavaScript in the browser (e.g. what the W3Schools page shows).  I am referring to actual script blocks located within the XSL during the transformation.</p>

Yes. It's browser dependant but you can use Javascript. There is a small but practical tutorial on w3schools.com. It's part of the XSLT tutorial.
The page:
http://www.w3schools.com/xsl/xsl_client.asp
The XSLT tutorial:
http://www.w3schools.com/xsl/default.asp
That site will be more helpful than myself. Good luck!