Those are my changes to Sir Trevor Editor Uploader, to allow me to upload the files directly to Amazon S3. It's just an experiment, so it's still a little messy, but it's working.

config_controller.rb

# As we cant make Secret Access Key open, this method generates a signature that will allow the user to upload this 
# file specifically 

  def sign_s3
    bucket_name = 'mailee-images'
    original_name = params['file_name']
    original_extension = original_name.split('.').last
    mime_type = params['file_type']
    object_name = Time.now.to_f.to_s.gsub('.','') + '.' + original_extension
    #TODO return unless mime_type is valid
    date = Time.now.utc.iso8601.gsub(/-|:/,'')
    amazon_headers = ["x-amz-acl:public-read","x-amz-date:#{date}"] # set the public read permission on the uploaded file
    string_to_sign = "PUT\n\n#{mime_type}\n\n#{amazon_headers[0]}\n#{amazon_headers[1]}\n/#{bucket_name}/#{current_client.id}/#{object_name}";
    signature = Base64.strict_encode64(OpenSSL::HMAC.digest('sha1',  APP_CONFIG['aws_secret_access_key'], string_to_sign))
    path = "https://s3.amazonaws.com/#{bucket_name}/#{current_client.id}/#{object_name}"
    r = {
      signature:  "AWS #{APP_CONFIG['aws_access_key_id']}:#{signature}",
      date: date,
      url: path,
    }
    render json: r
  end

uploader.js.coffee

#
#*   Sir Trevor Uploader
#*   Generic Upload implementation that can be extended for blocks
#
#*   modified by @pedroaxl to upload directly to S3

SirTrevor.fileUploader = (block, file, success, error) ->
  SirTrevor.EventBus.trigger "onUploadStart"
  uid = [block.blockID, (new Date()).getTime(), "raw"].join("-")

  block.resetMessages()
  callbackSuccess = (data, textStatus, jqXHR) ->
    SirTrevor.log "Upload callback called"
    SirTrevor.EventBus.trigger "onUploadStop"
    # once the PUT response from Amazon comes empty, I had to change this line to include the url
    _.bind(success, block) {file: this.url}  if not _.isUndefined(success) and _.isFunction(success)

  callbackError = (jqXHR, status, errorThrown) ->
    SirTrevor.log "Upload callback error called"
    SirTrevor.EventBus.trigger "onUploadStop"
    _.bind(error, block) status  if not _.isUndefined(error) and _.isFunction(error)

  $.ajax(
    url: '/config/sign_s3'
    #   before sending to Amazon using PUT, it makes a req to get the signature that will allow the upload
    data: {
      file_name: file.name
      file_type: file.type
    }
    type: 'GET'
    error: callbackError
    success: (data, textStatus, jqXHR) ->
      xhr = $.ajax(
        url: data.url
        contentType: file.type
        crossDomain: true
        data: file
        processData: false
        headers: {
          'Content-MD5': file.md5
          'Authorization': data.signature
          'x-amz-date': data.date
          'x-amz-acl': 'public-read' # set the public read permission on the uploaded file
        }
        type: "PUT"
        success: callbackSuccess
      )
      block.addQueuedItem uid, xhr
      xhr.done(callbackSuccess).fail(callbackError).always _.bind(block.removeQueuedItem, block, uid)
      xhr
  )