s3 shunked uploads with blueimp.fileupload

I'm implementing AWS chunked uploads using the blueimp plugin, and I've run into a problem in the event ordering.

Instead of Content-Range (presence of which causes Amazon to immediately throw a 403), S3 uses an upload ID and part number query parameters.

So before each chunk, I need to reach out to my signing service and change the url of the next chunk.

It doesn't appear that options.chunksend blocks the event system like options.add, so my next chunk is sent to the same URL as the first chunk (set in options.add), overwriting it.

How can I block the send of a chunk so I can change the url?