Eric, I like these kinds of calculations and analyses. I think your work calculations are sound. Although to be a total nerd, I am not sure the center of gravity of the body moves the same vertical distance that the barbell does. Another point is that work done other than against gravity (e.g., stabilization done by postural muscles) is neglected. But I think those things really are negligible in this case.
However, I think the main point of the calculations is to demonstrate that, all other things being equal, you can almost always do more work over the course of a workout with the lower momentary power output, done at a steady pace. The more the momentary power output fluctuates, the less the average power output and total work done will be.
You won't cover as much ground in twenty minutes of sprint intervals or walkbacks as you would running at a hard, steady pace. (If you could, people would run races by walking 100 yards, then sprinting 100, etc.) The steady pace workout will have a higher average power output, but that doesn't mean intervals are inferior.
If you did EDT with bodyweight, you could certainly match your Tabata power output. It's not Tabata vs Staley, it's bodyweight squats vs weighted squats.
Varying the intensity of the load, in this case the amount of weight, will have different training effects. Total power output isn't everything. (If it was we'd just run at a steady pace, at our lactic acid threshhold, and be done with it.) As you noted, a workout of all singles can be pretty paltry in terms of power output.
I still agree with your conclusion that the Tabata scoring rule is an excellent rule of thumb. A good, specific mark to hit before adding weight.