webpack-hot-middleware with separate host

.babelrc

{
  "stage": 0,
  "env": {
    "development": {
      "plugins": ["react-transform"],
      "extra": {
        "react-transform": {
          "transforms": [{
            "transform": "react-transform-hmr",
            "imports": ["react"],
            "locals": ["module"]
          }, {
            "transform": "react-transform-catch-errors",
            "imports": ["react", "redbox-react"]
          }]
        }
      }
    }
  }
}

package.json

{
  "scripts": {
    "start": "babel-node ./<path to devserver>/server.js"
  },
  "dependencies": {
    "react": "0.13.3",
  },
  "devDependencies": {
    "babel-core": "^5.8.25",
    "babel-loader": "^5.3.2",
    "babel-plugin-react-transform": "^1.1.1",
    "express": "^4.13.3",
    "react-transform": "0.0.3",
    "react-transform-catch-errors": "^1.0.0",
    "react-transform-hmr": "^1.0.1",
    "redbox-react": "^1.1.1",
    "webpack": "^1.12.2",
    "webpack-dev-middleware": "^1.2.0",
    "webpack-hot-middleware": "^2.4.1"
  }
}

server.js

import path from 'path';
import express from 'express';
import webpack from 'webpack';
import hot from 'webpack-hot-middleware';
import dev from 'webpack-dev-middleware';
import config from './webpack.config.dev';

const app = express();
const compiler = webpack(config);

app.use(dev(compiler, {
  noInfo: true,
  publicPath: config.output.publicPath,
  headers: {
    'Access-Control-Allow-Credentials': true,
    'Access-Control-Allow-Origin': 'http://<appserver hostname>:<appserver port>'
  }
}));

app.use(hot(compiler));

app.listen(<devserver port>, '<devserver hostname>', err => {
  if (err) {
    console.log(err);
    return;
  }

  console.log('Listening at <devserver hostname>:<devserver port>');
});

webpack.config.dev.js

import path from 'path';
import webpack from 'webpack';

export default {
  devtool: 'eval',
  entry: [
    'webpack-hot-middleware/client?path=http://<devserver hostname>:<devserver port>/__webpack_hmr',
    './<path to app entry point>'
  ],
  output: {
    path: path.join(__dirname, 'dist'),
    filename: '<public app filename>.js',
    publicPath: 'http://<devserver hostname>:<devserver port>/<public app path>/'
  },
  plugins: [
    new webpack.HotModuleReplacementPlugin(),
    new webpack.NoErrorsPlugin()
  ],
  module: {
    loaders: [{
      test: /\.jsx?$/,
      exclude: /(node_modules|bower_components)/,
      loader: 'babel'
    }]
  }
};

You have a syntax error in your package file: there is a comma after "react": "0.13.3" which invalidates the JSON.