It does.

  enum EventType {
     ...,
     EVENT_USER_START = 0xF0000000,
  };
  struct UserEventData { uint8 placeholder[64]; }
  struct Event {
     EventType type;
     union {
         UserEventData user;
     };
  }

If you're writing a good library, give the user the data they need, and let them handle it however they wish, don't force them int your framework.

That doesn't work if your library needs to handle these events somehow, not just transparently pass them around.

Typically with your pattern, you'd use a switch statement on the event type, but what do you do if it's in the user-defined space (type>=EVENT_USER_START)? You could add mandatory function pointers for relevant handlers in a header prefix, but now you're just reimplementing virtual methods. As is common in C, you end up making your own object-oriented language, only with more effort and not as good.

It does not work if two independent libraries depend on your library and create the same user EventType.