I just tried the WildFly Alpha 2 snapshot containing Narayana 5.0.0.M3 and get the error message below. The classes causing the error are:
@Path("/bestellungen")
@Produces(

)
//@RequestScoped is default for JAX-RS
@Transactional
public class BestellungResource {
@Inject
private BestellungService bs;
...
}

@Transactional(TxType.MANDATORY)
public class BestellungServiceImpl implements Serializable {
private static final long serialVersionUID = ...L;

@PersistenceContext
private transient EntityManager em;

// method invoking JPA / Hibernate
}

The fragment of server.log:
15:11:43,254 DEBUG [org.hibernate.SQL]
/* named HQL query Bestellung.findKundeById */ select
distinct abstractku1_.id as id2_8_,
abstractku1_.aktualisiert as aktualis3_8_,
abstractku1_.bemerkungen as bemerkun4_8_,
abstractku1_.email as email5_8_,
abstractku1_.erzeugt as erzeugt6_8_,
abstractku1_.file_fk as file18_8_,
abstractku1_.kategorie as kategori7_8_,
abstractku1_.nachname as nachname8_8_,
abstractku1_.newsletter as newslett9_8_,
abstractku1_.password as passwor10_8_,
abstractku1_.rabatt as rabatt11_8_,
abstractku1_.seit as seit12_8_,
abstractku1_.umsatz as umsatz13_8_,
abstractku1_.version as version14_8_,
abstractku1_.vorname as vorname15_8_,
abstractku1_.familienstand_fk as familie16_8_,
abstractku1_.geschlecht_fk as geschle17_8_,
abstractku1_.art as art1_8_
from
Bestellung bestellung0_
inner join
kunde abstractku1_
on bestellung0_.kunde_fk=abstractku1_.id
where
bestellung0_.id=?
15:11:43,254 WARN [org.hibernate.engine.jdbc.spi.SqlExceptionHelper] SQL Error: 0, SQLState: null
15:11:43,254 ERROR [org.hibernate.engine.jdbc.spi.SqlExceptionHelper] UserTransaction is not available within the scope of a bean or method annotated with @Transactional and a Transactional.TxType other than NOT_SUPPORTED or NEVER