Ok. I get the feeling you know what you are doing. I was going to suggest that approach (use a Doc library as a temporary staging location) but because of the complexity, I held off. But your thinking along the same line.
You should download the latest from github, since I have made some fixes but have not yet cut a new version.
Your approach should be:
1. Upload file to Doc library
2. Read the file from the doc library (I believe the copy service has a API method to do this). The file content (I think) will already be returned as BASE64 - but not sure.
3. Write that file to the List item (remember: you may have to check out the item if the list has checkout turned on)
4. Delete the temporary file in the doc library
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