Oh that's interesting. You probably still need to jump through hoops to figure out the right url for the grid square and level of detail you need. And the session parameter - so you still need to make the initial tile request to get the JSON.
Do a little googling and you can probably find code on github to take care of the "get me terrain for these coordinates" part for you. That's how I managed it a few years ago when I was programmatically downloading tiles from google earth without understanding how to convert from coordinates to their url scheme.
