app-schema does not recognise substitution groups of abstract types

...
Secondly, I'm trying to do something similar with the gsml:positionalAccuracy element. In this case, I want the resultant document to look like this:

<gsml:positionalAccuracy>
  <gsml:CGI_TermValue>
    <gsml:value codeSpace="urn:ietf:rfc:2141">urn:ogc:def:nil:OGC:missing</gsml:value>
  </gsml:CGI_TermValue>
</gsml:positionalAccuracy>

When I add the mapping to my MappedFeature.xml to do so, I get the following error:

gsml:CGI_TermValue is not a valid location path for type urn:cgi:xmlns:CGI:GeoSciML:2.0:CGI_ValuePropertyType.

In my understanding of the schema, CGI_TermValue should fit here as a result of a couple of substitution groups. gsml:positionalAccuracy requires a CGI_Value. CGI_TermValue or CGI_NumericValue substitute for CGI_PrimitiveValue which substitues for CGI_Value. Here are some relevant bits from the value.xsd:
____

<xs:element xmlns:xs="http://www.w3.org/2001/XMLSchema" name="CGI_TermValue" substitutionGroup="gsml:CGI_PrimitiveValue" type="gsml:CGI_TermValueType">

<xs:element xmlns:xs="http://www.w3.org/2001/XMLSchema" abstract="true" name="CGI_PrimitiveValue" substitutionGroup="gsml:CGI_Value" type="gsml:CGI_PrimitiveValueType">

<xs:element xmlns:xs="http://www.w3.org/2001/XMLSchema" abstract="true" name="CGI_Value" type="gsml:CGI_ValueType">

<xs:complexType xmlns:xs="http://www.w3.org/2001/XMLSchema" name="CGI_ValuePropertyType">
   <xs:sequence>
       <xs:element ref="gsml:CGI_Value"/>
   </xs:sequence>
</xs:complexType>

<xs:element name="positionalAccuracy" type="gsml:CGI_ValuePropertyType">
____

Is it possible that the whatever is doing the parsing here is not traversing these substitution groups? Is there (perhaps more likely) something that I might not have in place that would correct this issue? Maybe my namespaces are not correct or complete in the mapping file? Not really sure...

Thank you,
Ryan

				<AttributeMapping>
					<targetAttribute>gsml:positionalAccuracy</targetAttribute>
					<targetAttributeNode>gsml:CGI_TermValuePropertyType</targetAttributeNode>
				</AttributeMapping>
				<AttributeMapping>
					<targetAttribute>gsml:positionalAccuracy/gsml:CGI_TermValue/gsml:value</targetAttribute>
					<sourceExpression>
						<OCQL>'urn:ogc:def:nil:OGC:missing'</OCQL>
					</sourceExpression>
					<ClientProperty>
						<name>codeSpace</name>
						<value>'urn:ietf:rfc:2141'</value>
					</ClientProperty>
				</AttributeMapping>

29 Jul 14:46:37 ERROR [geoserver.ows] - 
java.lang.IllegalArgumentException: gsml:CGI_TermValue is not a valid location path for type urn:cgi:xmlns:CGI:GeoSciML:2.0:CGI_ValuePropertyType. gsml:CGI_TermValue ns: urn:cgi:xmlns:CGI:GeoSciML:2.0, CGI_ValuePropertyType properties: urn:cgi:xmlns:CGI:GeoSciML:2.0#CGI_Value
	at org.geotools.data.complex.filter.XPath.set(XPath.java:601)
	at org.geotools.data.complex.MappingFeatureIterator.setAttributeValue(MappingFeatureIterator.java:371)
	at org.geotools.data.complex.MappingFeatureIterator.computeNext(MappingFeatureIterator.java:555)
	at org.geotools.data.complex.MappingFeatureIterator.next(MappingFeatureIterator.java:453)
	at org.geotools.data.complex.MappingFeatureIterator.next(MappingFeatureIterator.java:82)
	at org.geotools.xml.Encoder.encode(Encoder.java:665)
	at org.geotools.xml.Encoder.encode(Encoder.java:557)
	at org.geoserver.wfs.xml.GML3OutputFormat.write(GML3OutputFormat.java:170)
	at org.geoserver.wfs.WFSGetFeatureOutputFormat.write(WFSGetFeatureOutputFormat.java:137)