Note: original text by @Antonio Vazquez (antoniov)

When enable sculpt mode in grease pencil, the toolbar shows a group of buttons that run a operator. The problem is the brush is not changed until the user press on the view-port. This is logic because the invoke event is not called.

The first consideration is that sculpt brushes are not real brushes, it’s only a UI representation of operator with different parameters, so we cannot use the “general” brush solution.