Note the usage of the scoped variables along with custom text in constructing the url of the script library
<xp:headTag tagName="script" rendered="true" loaded="true">
<xp:this.attributes>
<xp:parameter name="clientSide" value="true"></xp:parameter>
<xp:parameter name="src">
<xp:this.value>
<![CDATA[#{javascript:return applicationScope.get("senchaSourceBase")+"sencha-touch-debug.js"}]]>
</xp:this.value>
</xp:parameter>
</xp:this.attributes>
</xp:headTag>