The RESTAT integration API Participant interface is serialized during commit processing (in the method org.jboss.narayana.rest.integration.RecoveryManager#persistParticipantInformation) but the interface does not document this responsibility. The javadoc should say something like:

Participant implementations must be implement either {@link java.io.Serializable}or{@link PersistableParticipant}

Secondly, during commit processing RecoveryManager#persistParticipantInformation needs to throw an exception if the participant cannot be written to the object store. At the moment it just writes a TRACE message.