If procedural node creation is possible it is somewhere in C++. I would suggest reposting this question in that answerhub for the best results.
I am curious though, what kind of mechanic are you thinking about that would need such a macro? You could probably emulate the same kind of thing with Arrays. X Happens, then Array element is generated that runs a Switch to Y branches.
Essentially your basic branch setup will reach some fundamental limit of complexity, so that is how big your Enum would be. Then you procedurally generate the Array elements to choose between them. Your array could be an arbitrary length and accomplish the same thing as your proposed Branch idea.