Dear Nick,
I forgot about our discussion. Since Orthanc 0.7.5 (released in May, 2014), it is possible to modify all the instances of a patient in a single REST call. For instance:
This line would modify all the instances of the patient whose Orthanc ID is "6ab46a95-3c7b782d-757bc7b7-9a3c0ce6-e09b1a3c", replacing her name with "Sample patient name".
Please note that, in this case, you have to replace the (0010,0020) "PatientID" tag for Orthanc to accept this modification: This is to prevent the merging of patient data before and after anonymization, if the user does not explicitly tell Orthanc to do so.
HTH,
Sébastien-