I had success this morning with uploading a document to the user space. It looks like I was missing the (empty) updateNodeRef parameter. So the final list of payload parameters should be:
filedata: filename="somefile.txt" Content-Type:text/plain
siteid:
containerid:
uploadDirectory:
description: user home upload
contentType: cm:content
thumbnails: doclib
overwrite: false
destination: workspace://SpacesStore/node_number_for_user_home_directory
username:
majorVersion:
updateNodeRef: