Add the sidebar/widget editor to the post edit screen?

I’m sure there is a more elegant approach (I’d love to know about!). Anyhow: I’d like to offer a solution.
You could create a default sidebar with default widgets and a custom sidebar with all available custom widgets. Then you use a combination of the following plugins: simple fields and widget logic to only show a selection of widgets.

How to:

1. Fill the default sidebar, add a condition that shows this sidebar only if a certain metadata value is NOT set.
2. Fill the custom sidebar with all optional widgets, add a condition to every widget to show up only if a certain metadata field is available
3. Set up simple fields to write the conditional metadata

This way works quite well for me, though it is not very pretty.

Hope it helps or gives ideas for further extension.