... and other macros could invoque it as well, is that your idea? This should work at first glance.
Now, there is evolution pressure on the memberwise init: it is regularly the topic of feature requests, evolution discussions, or pitches. Strictly locking the memberwise init inside the feature set of macros would seriously limit its evolution space, don't you think? For example, some "issues" listed in this old thread State of the Memberwise Initializer are still relevant (access control and classes).
The memberwise initializer is one of the most crucial user-facing features of the language. Its ergonomics are paramount, and should be enhanced, not restricted. I understand that macros were introduced in order to address the need and desire to send inner compiler features to userland (among other reasons). But it's important to protect a few key areas - including the memberwise init IMHO. That's the idea I suggest the Language Steering Group to contemplate.