<xsl:value-of select="document('bool12.xml')/Data/person/name[../id='12']"/>
will do it. But
you should check how your XSLT processor implements document() function. Most XSLT processors are DOM-based, it may happen that your file will be read as a whole anyway, regardless of how you implement search. I read that Saxon works with SAX, maybe you will have to use it, if you do not already.