I done this before. At the time we were at 7.4 I believe. We are at 7.8 now, some changes may have occurred. I created a xml block and I created a xslt format in the output. The format is pulling the data but it is not applying the css. I put the same html code in a xhtml block and it works pretty. I am trying to use a xml file to manage my external site urls, used through out the site.

xml block and format - screen shot (menu links are off not applying the css)
see file xml-block.png

xhtml block - screen shot (menu is pretty)
see file xhtml-block.png

XML BLOCK CODE:

<system-xml>
    <LINK>
        <BOOKSTORE>http://ncat.bncollege.com</BOOKSTORE>
        <BRANDING>#</BRANDING>
    </LINK>
</system-xml>

XSLT Format CODE

<?xml version="1.0"?>
<xsl:stylesheet
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
    <xsl:template match="LINK">
        <!-- main nav -->
        <nav aria-label="Main Navigation" class="main-nav" id="main-nav" role="navigation">
            <a class="switcher" href="#">Menu
                <i class="fa fa-bars"/>
            </a>
            <ul>
                <li>
                    <a href="{BOOKSTORE}" onClick="ga('send', 'event', 'MAIN-NAVIGATION', 'click', 'MERCHANDISE');">MERCHANDISE
                        <i class="fa fa-shopping-cart"/>
                    </a>
                </li>
                <li>
                    <a href="/125/history" onClick="ga('send', 'event', 'MAIN-NAVIGATION', 'click', 'HISTORY');">HISTORY
                        <i class="fa fa-clock-o"/>
                    </a>
                </li>
                <li>
                    <a href="/125/events" onClick="ga('send', 'event', 'MAIN-NAVIGATION', 'click', 'EVENTS');">EVENTS
                        <i class="fa fa-calendar"/>
                    </a>
                </li>
                <li>
                    <a href="/125/publication" onClick="ga('send', 'event', 'MAIN-NAVIGATION', 'click', 'PUBLICATIONS');">PUBLICATIONS
                        <i class="fa fa-book"/>
                    </a>
                </li>
                <li>
                    <a href="/125/gallery" onClick="ga('send', 'event', 'MAIN-NAVIGATION', 'click', 'GALLERY');">GALLERY
                        <i class="fa fa-share-alt-square"/>
                    </a>
                </li>
                <li>
                    <a href="site://NCAT.edu/giving/give-now" onClick="ga('send', 'event', 'MAIN-NAVIGATION', 'click', 'GIVE-NOW');">GIVE NOW 
                        <i class="fa fa-gift"/>
                    </a>
                </li>
                <li>
                    <a href="/125/map" onClick="ga('send', 'event', 'MAIN-NAVIGATION', 'click', 'CAMPUS-MAP');">CAMPUS MAP
                        <i class="fa fa-map-marker"/>
                    </a>
                </li>
            </ul>
        </nav>
        <!--/ main nav -->
    </xsl:template>
</xsl:stylesheet>