IIRC HDMI is one-way and touch-screens send touch information back via USB or some other connector. I can't see any wording in the specs that implies this has USB host capability, so unless I'm wrong there it it wouldn't be able to doe touch that way.
You could do it over bluetooth, but at this point you're making a specialized display and you may as well just plug into the Cotton Candies USB port to both power it and pass in the touch screen HID data.
The HDMI connector has I2C on there. You could use that to communicate with a touchscreen and other peripherals. Since the display acts a slave only you would need to actively poll the screen all the time for touch events, but it's not that much overhead.
You could send back touch events over the HDMI audio return or Ethernet channels.