Readonly questions are tagged in the XForms binding. Values in the <response> element are ignored on submission.
QUESTION "Text question (read-only)"
READONLY TRUE
DFT "dft"
ANSWER text_variable
<xsd:schema targetNamespace="http://example.info/types">
<xsd:simpleType name="question0">
<xsd:restriction base="string" />
</xsd:simpleType>
</xsd:schema>
<xforms:instance>
<response xmlns="">
<question0>dft</question0>
</response>
</xforms:instance>
<xforms:bind nodeset="question0" id="question0" type="type:question0" readonly="true()" />
<itp:question>
<xforms:input ref="question0">
<xforms:label>Text question (read-only)</xforms:label>
</xforms:input>
</itp:question>