What are the specs of your laptop? How old is it? Most laptops have overheating protection that will shut down the system before it gets damaged. So it is probably fine to try it and see if there are any problems. It would of course be best to be plugged-in when using the laptop for Terragen.
Another option to consider is keeping your home computer online and powered-up at all times, install something like Google Drive or Dropbox on it as well as on your laptop, then when you go out with your laptop you can setup and design a scene, but send it back to your home system for rendering. You can use remote desktop software like TeamViewer to initiate a render on your desktop from the laptop.
- Oshyan