Okay, got you @wyclif. The code I committed can be further extended to support your use case of passing identifiers/uris. I think it deserves a separate issue.
As for now we have support for passing base64 encoded binary data in json or posting a file as is through the form-data approach. One can also fetch binary data stored on the server as is at /obs/{uuid}/value (the link is provided in the /obs/{uuid} response under the value field).