As far as the JPA contracts it was decided to rely on casting rather than these hint hacks. So to control this per-parameter, you'd cast the parameter as obtained from the javax.persistence.StoredProcedureQuery to org.hibernate.jpa.spi.StoredProcedureQueryParameterRegistration:

StoredProcedureQuery query = ...;
( (StoredProcedureQueryParameterRegistration) query.getParameter( "firstArg" ) ).enablePassingNulls( true );
...