In order to have the drawing styles at menus and toolbars we need access to it. There exists the UNO command StyleApply with the attributes Style <string> (set to "Filled Blue", for example) and FamilyName <string> which needs to be added.
Rough code pointer to start: https://opengrok.libreoffice.org/xref/core/sw/source/core/unocore/unostyle.cxx?r=4d502ef3#1086
@Maxim: Is that enough for being a qualified easyhack? (There are many occurrences for StyleFamily)
The problem is deeper than adding something to UI. There exists no special style family for frames but frame and shape styles both belong to the family 'graphic'. The distinction between frame and shape is done by the fact, that the automatic style which is referenced by the object has a parent style in case of a frame and does not have a parent style in case of a shape. The parent style is a custom style and visible in the UI.
Second problem is, that using custom styles is not implemented for shapes in Writer. Meaning, if you directly refer a custom style in the draw:style-name attribute of an object, it will make the object to a Writer-object if possible and will be ignored otherwise.
You can see what problems this has looking at the attempt to consolidate the features of Writer-images and Draw-images. Similar hard problems will occur with Writer-frames and Draw-textboxes.
These problems are homemade and have nothing to do with the ODF specification.
All together I classify it has hard task.
Could imagine a intermediate solution is possible.
It might be possible to solve a sub-task first: Implement the use of custom styles for a subset of shapes. Especially leave out those, which are written with draw:frame in file format. But that is still not an easyhack.
What about adding this issue to the GSOC's ideas page ?
(In reply to Xisco Faulí from comment #4)
> What about adding this issue to the GSOC's ideas page ?