Obtaining the water temperature from a heatpump using a microcontroller
By the spec sheet it looks like the controller has some sort of remote control port on it, though the documentation is vague in that part. You may be able to figure that out, maybe by a phone call to the manufacturer, and use that.

Failing that, figuring out which probe is the one you want, and how it works, and tap into that using a suitable high impedance to not mess things up. 

If you can not figure out that probe then add another.

The camera idea is cute though.