The problem is with modern diesel engines is that they are very efficient and don't produce much heat. Our engines don't get up to operating temperature in the winter mornings due to the short distances we travel. His demonstrator car is an old 1.6 diesel Golf.
I find that the engine looses its heat anyway being parked up, so you may as well put that heat to some use. How good it would be is questionable. He would need a pump to circulate the water from the engine, through the heat exchanger and back to the block.
In the kit he shows an auxillary pump. On the video on the beach he shows a small pump in the water tank at the back, but that's not it. This point isn't very clear. What ever the case is, in a window cleaning application we would need a 12v circulation pump as we couldn't leave the engine running. (Your internal heater matrix is a water to air heat exchanger. If you switch the engine off but leave the circulation fan running, it doesn't take long before the air coming out gets cold.)
The water circulating through the engine will be treated with antifreeze so there wouldn't be a problem there. However, the water in the second circuit of the heat exchanger will be pure water. Any water in there will freeze and damage the heat exchanger. Even with antifreeze, our washer bottle always seems to freeze at some point during winter. Even if the heat exchanger was drained down, the passages in the heat exchanger are very small and I doubt you will get every drop of water out. That would be a big negative for me.