Above, I should have pointed out the importance of FunctionReturn extending ParameterRegistration. Essentially it allows access to the return value. Unlike JPA StoredProcedureQuery, Hibernate's native ProcedureCall/ProcedureOutputs allow access to parameter outputs via the parameter memento:

    ...
    public <T> T getOutputParameterValue(ParameterRegistration<T> parameterRegistration);

Having FunctionReturn extend ParameterRegistration allows the FunctionReturn to be passed back in here to retrieve the return value. The other (cleaner, imo) option is to simply define a new method:

    ...
    public Object getFunctionReturn();