We have done this. We created our own report in the list that combines the single level BOM report with the related Documents table at the bottom. Technically, documents should not be part of the BOM unless the document is something physical, like a printed manual to be included in a ship set. Customizing BOM reports was added to the customizer's guide. Including the documents table was just copying in the JSP block to render the table for related documents.
Antonio Villanueva
Sr. Software Engineer
Goodrich - ISR Systems