Durn! This system just swallowed my reply. Grumble, will try once more.
I wrote "I don't understand why ..."
But now I think I can guess ...
The lab126 kernel was build without binfmt_misc selected in the configuration.
The "usual" presumption is that will cause only the module code not to be built.
My new guess -
That config option eleminated some of the supporting /proc code also.
Which would mean that the way you found to make it work, as shown in your top post, is the way it has to be done.