Okay, but what about "either a pointer to a status monitor or some estimate of when the user can expect the request to be fulfilled"?

Regarding the upload ID, it depends how it is generated.  It sounds like the server generates it in response to the first uploaded part, in which case I expect it has to be this way.  That's fine.

Incidentally, this is how the REST interface for ArcGIS handles it (with addItem?multipart=true, addPart, commit; sounds more like RPC than REST): 
http://resources.arcgis.com/en/help/arcgis-rest-api/index.html#//02r30000008q000000
http://resources.arcgis.com/en/help/arcgis-rest-api/index.html#//02r300000094000000
http://resources.arcgis.com/en/help/arcgis-rest-api/index.html#//02r30000008t000000

Uploading a file in multiple parts seems to be outside the bounds of normal atomic REST, so I suppose something has to bend, whatever we do.