I've been playing with browser uploads for a few weeks now (including S3) and was too worried about the complexities of uploading to S3.
I realized that there's nothing really fancy going on when uploading to S3. That's only true since Amazon added CORS support. Just add a few hidden form fields and you're good to go.
You might want to take a look at our Webscript example for doing this: https://www.webscript.io/examples/s3upload. (You can ignore the download part.) It should be trivial to use Dropzone.js instead of the normal <input> in the example HTML.
I realized that there's nothing really fancy going on when uploading to S3. That's only true since Amazon added CORS support. Just add a few hidden form fields and you're good to go.
Properly populating those fields can be a bit tricky, however. This gem will does a great job and should give you a head start: https://github.com/waynehoover/s3_direct_upload