> The most straightforward solution to this is the one that D employs, whereby A(B) is syntactic sugar for A.B() if the former is not otherwise valid.

Isn't the D solution actually that:

  A.B(...)

is syntactic sugar for:

  B(A,...)

if A.B() is not specifically defined.

(Yours both flips the elements and reverse the direction of sugaring, as I understand it.)