Hello Manjeet,
In my opinion i strongly feel that Delegating call to various BE systems through SAP__Origin parameters by passing system alias is nothing to do with mapping.
I have seen scenarios where there is no input mapped and no in put is passed as part of filters/keys, still MOC concepts is working when we just send System Alias name in SAP__Origin parameters as part of filters/keys.
I feel something else is missing so that its not working.Its not about mapping input or not mapping input.
Regards,
Ashwin