Using Draw objects in Calc and Writer should allow access to its styles.
This is not only a problem of extending the style&formatting dialog by a category "shape style". It goes deeper. Some years ago the decision has been made for Writer, that the fact, that no custom styles exists for shapes is used to distinguish a frame in Writer (those which can have nearly arbitrary content) from shapes (e.g. a text box). Such distinction does not exist in file format.
*** Bug 115704 has been marked as a duplicate of this bug. ***
*** This bug has been marked as a duplicate of bug 89369 ***