こんなイメージ

writablestream.coffee

Stream = require 'stream'
class Writable extends Stream.Writable
  constructor: (@socket, options = {}) ->
    super options

  write: (chunk, encoding, callback) ->
    @socket.emit 'data', chunk.toString(encoding)

  end: (chunk, encoding, callback) ->
    @socket.emit 'data', chunk.toString(encoding) if chunk
    @socket.emit 'end'

io.sockets.on 'connection', (socket) ->
  stream = fs.createReadStream 'package.json'
  stream.pipe new Writable socket