Itemset with some items bound to readonly nodes

form.xml

<xh:html xmlns:xh="http://www.w3.org/1999/xhtml"
      xmlns:xf="http://www.w3.org/2002/xforms"
      xmlns:xxf="http://orbeon.org/oxf/xml/xforms"
      xmlns:ev="http://www.w3.org/2001/xml-events"
      xmlns:xs="http://www.w3.org/2001/XMLSchema"
      xmlns:fr="http://orbeon.org/oxf/xml/form-runner">
    <xh:head>
        <xf:model>
            <xf:instance>
                <choice/>
            </xf:instance>
            <xf:instance id="choices">
                <choices>
                    <choice label="One" value="1"/>
                    <choice label="Two" value="2"/>
                </choices>
            </xf:instance>
            <xf:bind ref="instance('choices')/choice[2]" readonly="true()"/>
        </xf:model>
    </xh:head>
    <xh:body>
        <xf:select appearance="full" ref=".">
            <xf:itemset ref="instance('choices')/choice">
                <xf:label ref="@label"/>
                <xf:value ref="@value"/>
            </xf:itemset>
        </xf:select>
        <xf:input ref="instance('choices')/choice[2]"/>
    </xh:body>
</xh:html>