Some progress on thebug. The aws library now includes the multipart API. However, when I dug into it, it looks like the API needs some changes to get the ETAG of each uploaded part. Once that's fixed, git-annex should be able to support S3 multipart uploads, although I think that git-annex's own chunking is better in most situations -- it supports resuming uploads and downloads better. The main use case for S3 multipart seems to be using git-annex to publish large files.
Also, managed to get the backlog down from 100 to just 65 messages, including catching up on quite old parts of backlog.