Multipart is not at all impossible from a webapp (depending on your browser requirements). FileReader API is available across all browsers (except IE < version 10, which I admit is a big problem). For IE, you can resort to some other solution (perhaps Silverlight, or Flash?)
I will second this. At the bottom of this article there is a reference the article I wrote a while ago when Amazon released this feature. I was planing on doing multipart uploading at that time using the FileReader but there was a bug in the way S3 did CORS so I didn't want to continue until that was fixed. They fixed it and I never came back to it. Maybe be a good time to try it again. Resuming a partial upload seemed like a good win to me.
