Many years ago, at a university club, an instructor said that some standards group studying the mechanics of shotokan were recommending a change in the augmented block (morote-uke). Instead of placing the knuckle surface of the supporting hand against the inner elbow of the blocking hand, one should place the hammer fist striking surface aginst the inner elbow. I have not seen this anywhere else, but both alternatives seem to help. Was discussion of this a thing at one time, and it just didn't get traction?
but both alternatives seem to help
It depends on the application of the "block".
When you take a technique out of a kata, you also remove the context of its intended use. In general, I would take a look at how other styles perform the same kata; usually there is a reason that a particular body shape is used and without reference to what it's for, it's very easy to come to spurious conclusions.
My recommendation is to look at the kata you find the block in and the surrounding techniques to understand how the movement is really being used.
In short, context matters.
If you consider uchi-uke and morote-uke as arm manipulation techniques then the closed fist of the supporting hand may just be holding uke's wrist, and knuckles to elbow would more closely resemble reality.
I suspect this is why it hasn't really spread.