Wow, I would not want hollow pedal extenders.
I own pedal extenders from KneeSavers, Sunlite, and Aseparts that I use on a variety of bicycles and unicycles. Although I weigh about 250 pounds and tend to mash the pedals more than I should, I have had no problem with any of these. Even though I haven't conducted a specific stress analysis, as an engineer I don't see any problem with the hollow shaft Aseparts design.
The majority of the strength of this kind of component will be in the outside skin of the extension so a solid core doesn't offer as much additional strength as one might otherwise think. Frankly, I am more concerned with the large step and resulting stress concentration of the other designs. In any event, the choice of material and heat treatment will have a greater effect on the yield strength than the differences between any of these geometries.
Overall I am happy with the Aseparts version and I plan to buy a few more. Their price is very reasonable, it is made in America, and you can almost buy all three sizes for the price of one pair of KneeSavers. I will add though that I think it is critical that any leverage enhancing component like this be kept tight and I would avoid using any washers that might reduce the threaded coupling area and/or aggravate the strain.