Hi Shakeer
As you already know you have a very specific req so you have to go for development but you can use report SOLMAN_UNUSED_DOCUMENTS
this se38 report help us to locate the documents which are still in KW of solman but de-linked to projects. Thus you can use this report or modify this to as per your requirements which will reduce the efforts of building it from scratch.
In addition you can check the logic of Solar_eval which is used for finding all documents assigned to a specfic project
thus above will help you to get there.:)
Hope it helps
Regards
Prakhar