That is one of my favorites.
There was a test of that several years ago (in a duplex, maybe in Detroit?).
The supply to the water heater went around the drain, so it was not heating water for the toilet.
The water heater was electric with the typical 2 elements, 1 high power, 1 low power.
The high power element never turned on during the testing and logging duration.
The drain and coils were copper. Seems like the drain was 4"x4', and the coils were 3/4"?
Coil was just wrapped the drain, and not even insulated! Seems to me like it would have been more efficient if the coil was soldered to the drain, and the whole thing insulated!
The drain must be as vertical as possible so the warm grey water spreads out over the max surface area of the inside of the copper drain. It helps the heat transfer and slows the rate of fall, making it more efficient.
Trust me, if my shower was not over a crawl space, I would have made one of those as soon as I saw the reports.
G-