It was config issue from my side I did placed the the DD Module builder pipeline in the last.
<add type="Sdl.Web.Modules.DynamicDocumentation.Mapping.ModelBuilder, Sdl.Web.Modules.DynamicDocumentation" />
Important to note:
In the modelBuilderPipeline section, add the Dynamic Documentation model builder to the pipeline, ...
The DXA Container Framework was more or less abandon when the region concept was introduced in Sites 9.0. If there are still are some interests I could look into reviving the framework again and adapt it the region concept + upgrade it to DXA 2.2. It could potentially serve as a gap filler until user defined regions are available in Tridion Sites.
User Defined Regions indeed seems to be a popular feature. See also SDL Ideas (ensure to leave a vote & comment there): https://community.sdl.com/ideas/sdl-tridion-dx-ideas/i/sdl-tridion-sites-ideas/allow-editors-to-insert-regions-into-a-page
It has been high on the prioritized list of candidate features for quite a while, but unfortunately not high ...
The solution I mentioned in the question seems to be the answer. I have created a pull request in order to introduce the fix and potentially get hotfix out of it.