Is it possible to annotate PDFs displayed in Safari browser (setting Preview as the default viewer)?
There is no special "Preview plugin" for Safari.
Safari – just like Preview.app – uses PDF Kit, an OS X framework, to display PDFs. However, Safari can not create annotations on these PDFs, as it just doesn't have the implementation for creating annotations. Apple would have to re-program Safari to add these features and the necessary interface for Safari to edit PDF documents.
Or you'd need to use another plugin that actually provides this functionality (none that I know of).