Yes, orthanc can be acted as a relay node. You need to write a c++ plugin (or python) to convert WADO/ QIDO into Dicom protocol. Steps maybe are:
1/ When Wado/ QIDO request arrives in Orthanc, the plugin parse the http content to get the studyInstanceUID,
2/ Check if studyInstanceUID is existed in Orthanc DB
3/ If it has, then using Wado/ QIDO API from Orthanc and reponse with the result
4/ If it does not have then using C-Move API from Orthanc API to retrieve the Study from DCM4CHEE