The xinclude tag does have xpointer support as per http://www.w3.org/2001/XInclude which helps include specific tags from external xml's. Please support xpointer in solr as well. This will help in modularization and reusability.
<xi:include href="child.xml" xpointer=(//root) xmlns:xi="http://www.w3.org/2001/XInclude">
1. "The Xerces parser, used by default in Solr, doesn't support the xpointer="xpointer()" scheme." from https://wiki.apache.org/solr/SolrConfigXml#XInclude