Thanks for your explanations. 

It sounds, that there is no way to fix this issue officially. Only the meta-class workaround is left, which is - as you highlight - really just a workaround.

I anyway always wonder, why functions, which are methods, do not hold a reference to the class, which they belong to. It could help here and second, this would be a good way to differentiate if a function object is a method or not.