Do you have any working examples of the AI needed to grade the unstructured writing?
Getting Moodle to add a given grade to an assignment is probably not that difficult. Getting a system to look at, evaluate and provide a reasonably accurate grade for a piece of unstructured writing that goes beyond a sentence that can pick out a couple of key words/phrases will require extensive linguistics, model answers, plaigiarism detection (or integration with existing plaigiarism plugins) and so much more besides.
The direct answer to your question is that submission and feedback plugins are just that, plugins in the assignment system and you can look at several of them in core or in the Moodle pluginsdatabase to see how they work. You would need, I think, to create a new submission plugin to accommodate the work you are proposing.