custom jquery-fileupload processAction: Found a simpler way to do what I wanted, but this could be useful reference for times when you want to pre-process a file when user adds it to the queue.

custom jquery-fileupload processAction.js.coffee

#
# Custom processActions
#
# Prepend to the default processQueue:
$.blueimp.fileupload.prototype.options.processQueue.unshift(
    {
        action: 'prepS3Upload',
        # Use the action as prefix for the "@" options:
        prefix: true,
        fileTypes: '@',
        maxFileSize: '@',
        disabled: '@disablePrepS3Upload'
    }
);


$.widget('blueimp.fileupload', $.blueimp.fileupload, {
#    options: {},
    processActions: {
      prepS3Upload: (data, options) ->
        dfd = $.Deferred()
        file = data.files[data.index]
        $.ajax({
          url: data.uuid_generator_url,
          type: "GET",
          dataType: "json",
          contentType: "application/json; charset=utf-8",
          success: (response) ->
            $('#fileupload').find('input[name=key]').val(response.key);
            $('#fileupload').find('input[name=policy]').val(response.policy);
            $('#fileupload').find('input[name=signature]').val(response.signature);
            data.success_action_redirect = response.success_action_redirect
            file.uuid = response.uuid
            file.s3_key = response.key
            dfd.resolveWith(this, [data])
          ,
          error: (response) ->
            data.files.error = true
            file.error = "Could not generate UUID"
            dfd.rejectWith(this, [data])
        })
        return dfd.promise()
    }
})