What a great quality post! I did not respond to it right away because I wanted to make sure I had time to see everything.
I really like that idea, and I've seen such planks in the past.
One thing to note is that when the guy's setup failed, it wasn't because the torque was too high, but because it places too much weight outside of the setup's base of support. If you were to put the straps close to one extremity, all you would need is a plank that extends far enough so that your body (or most of it, but let's say your whole body for the sake of safety) stays over the plank. That way, even if you were to do those exercises with 300% BW added... The plank will never flip like that. It can break. But not flip.
Here is the design I suggest:
Make the board thick, and long enough so that your body stays inside of it. You can make it shorter, but you'll have to calculate how much weight there is over the board vs outside, and at what height and leverage. So just make it long enough. I recommend glueing and compressing 2x2s or 2x4s side by side to make it super thick and strong, then planning it so it's perfectly flat, and then cutting the slots for the straps at the desired places. I chose to place many slots because it can be nice to have different straps for each foot/leg so that they're not just squeezed together. Notice that from profile, the bottom of the plank has a different profile where the slots are located. This is to leave space for the straps or ropes to pass underneath.
Also different designs would be cool. Example:
-A shorter board could be made if retractable stabilizers could be added. I can think of many different designs. Could be one or two beams that slide out from the board at the opposing end of the straps. The board could also have dowels sticking out on each side, and two beams with holes in them could be inserted like Lego blocks then strapped to prevent them from falling out if the dowels are short. The whole point of the stabilizers would be to make the plank's base of support longer so that the weight doesn't tip outside of it.
-Make notches underneath the board at either extremity, or place hooks, so that the board could be hooked onto stall bars to modify its inclination/declination. I can see many different possibilities. Even if someone doesn't have stall bars, they could certainly build a small incline ladder onto which the plank could hook to vary its extremities' elevation. Or the plank could have short dowels sticking out on each side, and the structure could have notches on each side in which the dowels could rest. Like that, it would be less expensive than building a ladder using thick dowels, because those are expensive. If I have to build such a contraption, that's probably what I'd do. A thick but short dowel on each side of each extremity, and a foldable triangular structure with notches on each side.
-An other choice could be to add rotating arms at each end of it, and make them lockable like beach chairs to vary the height. But I think it's a fairly limited design, because it gets complicated, makes the board much more bulky, and you can only make those arms so long. I imagine you could make those arms extensible to create bigger differences in height, but it gets even more complicated. Simply making the plank inclinable to fit stall bars is good enough.
-Inclination could be changed by using straps and a door. Find a solid door, have the straps locked above the door using an object too thick to make it through on the other side, and hook the board onto the strap. Advantages: very simple and doable anywhere for cheap. Disadvantage: needs a plain wood door to avoid damage, places stress on the hinges, and might not be the most reliable in terms of solidity, so may be prone to dropping. So, could be dangerous.
-A 2x4 frame can be built, and plywood or boards can be used to cover the top. But the wood around the slots need to be thick and solid. I suggest using 2x4 or 2x2 for this section. Various combinations are possible, such as simple planks held together with 2x4s or 2x2s at a couple different places. One nice possibility would be to laminate wide planks (like 1x6s) in two layers, with two different directions. Could make a nice 2" thick plank that's 2 feet wide and 6 feet long pretty easily. The slots still would need to be reinforced, unless hard wood is used. Do NOT use pine, it will absolutely crack.