There is nothing strange or inherently unsafe about a backside approach to a frontside flare landing on a lake.
Yes, there is!
ALL backside [of the power curve] approaches to any surface are inherently unsafe. They are inherently unsafe because of the simple fact that you are flying on the back side of the power curve, and this means that power is
required for sustained flight in that configuration, and more power may be required to flare (arrest your rate of descent). If you loose engine power, or mis use power, a safe landing at all is unlikely, let alone where you planned to touch down.
Core is onto something, because he is acknowledging that it may be necessary to add power to flare and land. This is a possibility for a "too slow" approach power on or off. If you don't have that power available to arrest the descent (the flare), you won't stop going down before you hit the surface. When you are going down toward the surface with an intent to land gently on it, there will be a requirement to trade energy to accelerate the plane upward away from the descent angle to stop going down, and land. In an airplane, that energy will come from one of only two places: airspeed you stored in excess of stall speed (* plus a bit), or engine power. If you don't have engine power available, you're relying on only airspeed in excess of stall speed. *by the way, if the descent angle is steep, stall speed at the mid point of your attempt to flare may be a little more than a 1G stall, because you're pulling a little more than 1G in the flare, you're accelerating the plane upward - pulling G. It's only a tiny amount of G, my experiments show that even for a sharp flare, you're pulling 1.05G, but if you only had 1.1G worth of stored excess energy in speed, and then you pull, you slow down as you pull, and the G goes up a little, they meet, and you stall. When you stall then, you keep going down, and the ground is right there - you're going to hit hard.
This is not about touching down where you planned following an engine failure, this is about simply surviving the touchdown wherever it happens! Yes, on a few occasions I have flown backside approaches in Cessnas to assure that I got into the short runway/lake. In each case, I did it with the understanding that this was a much greater risk landing. Those approaches were always flat, not steep. The flat approach meant that it was only a very slight change to flare and touch down, rather than having to add more power to arrest my descent angle. - the approach was already risk enough!
This is exactly why, following the takeoff of your choice, the plane should be allowed to accelerate to a suitable faster climb speed unless there are obstacles. If flown needlessly slowly in the climb, a sudden engine failure may result in the pilot never achieving a good glide speed, and arriving to the surface with a back side of the power curve speed, and no power to arrest the descent.
So, new pilots, you've read Corethatthermal state that he's not
suggesting backside of the power curve approaches, just
asking about them. The answer to Core, and all pilots is: NO, don't do it! Fly the speeds and techniques recommended by the aircraft manufacturer.