Don't know if anyone caught this one yet..
<contribution configuration-id="hivemind.state.ApplicationObjects">
<state-object name="registration-data" scope="session">
<create-instance class="org.example.registration.RegistrationData"/>
</state-object>
</contribution>
Should be
<contribution configuration-id="tapestry.state.ApplicationObjects">
<state-object name="registration-data" scope="session">
<create-instance class="org.example.registration.RegistrationData"/>
</state-object>
</contribution>