As this is what I am struggling with too, may I continue this thread?
Seems like Elements facility in GP makes this possible: there I can use Sidebar blocktype and some location rule so it replaces the standard sidebar.
The problem I have: it’s not possible to have a widget in the Sidebar Element that selects the newest posts *only* from the category chosen in the Element Location.
Even with the help of the Category Posts plugin there’s no succes.
So then I installed the Content Aware Sidebars plugin and deactivated the Element, but this way I loose the editing facilities of Elements and am restricted to working with the widgets section.
It feels as if I’m too confused or ignorant about the precise functions that are in play here.
Any possible tip or correction from your side?