Disabling IDREF checking in Xerces C++ SAX parser

Is there a way I can disable ID/IDREF checking when using Xerces C++ SAX parser?

What I need is to be able to parse data files containing references (IDREF) to previously loaded data (ID).

For example, let's say I define books and bookmarks as follows:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema targetNamespace="urn:ns1"
       xmlns:ns1="urn:ns1"
       xmlns:xs="http://ift.tt/tphNwY"
       elementFormDefault="qualified">

  <xs:complexType name="book_type">
    <xs:attribute name="title" type="xs:ID" />
  </xs:complexType>
  <xs:complexType name="bookmark_type">
    <xs:attribute name="title" type="xs:IDREF" />
    <xs:attribute name="bookmark_type" type="xs:nonNegativeInteger" />
  </xs:complexType>

  <xs:element name="library">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="book" type="ns1:book_type" maxOccurs="unbounded" />
      </xs:sequence>
    </xs:complexType>
  </xs:element>

  <xs:element name="currently_reading">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="book" type="ns1:book_type" maxOccurs="unbounded" />
        <xs:element name="bookmark" type="ns1:bookmark_type" maxOccurs="unbounded" />
      </xs:sequence>
    </xs:complexType>
  </xs:element>

</xs:schema>

I then extend the previous grammar:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema targetNamespace="urn:ns2"
       xmlns:ns1="urn:ns1"
       xmlns:ns2="urn:ns2"
       xmlns:xs="http://ift.tt/tphNwY"
       elementFormDefault="qualified">

  <xs:import namespace="urn:ns1" schemaLocation="ns1.xsd" />

  <xs:element name="today_s_bookmarks">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="bookmark" type="ns1:bookmark_type" maxOccurs="unbounded" />
      </xs:sequence>
    </xs:complexType>
  </xs:element>

</xs:schema>

Is there any way to tell Xerces to ignore book ID missing in documents defined using the second schema?