It requires the following features: xslt-1 or xslt-1-output-base-uri.
The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:xslt version="1.0" output-base-uri="http://base.uri/doc.xml">
<p:with-input>
<doc/>
</p:with-input>
<p:with-input port="stylesheet">
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
<xsl:template match="doc">
<xsl:copy-of select="."/>
</xsl:template>
</xsl:stylesheet>
</p:with-input>
</p:xslt>
<p:identity>
<p:with-input>
<uri>{p:document-property(., 'base-uri')}</uri>
</p:with-input>
</p:identity>
</p:declare-step>
Schematron validation
<s:schema xmlns="http://www.w3.org/1999/xhtml" xmlns:s="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt2">
<s:pattern>
<s:rule context="/">
<s:assert test="/uri">The document root is not 'uri'.</s:assert>
<s:assert test="/uri/text()='http://base.uri/doc.xml'">Text child of uri is not 'http://base.uri/doc.xml'.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
