Hi,
I don't believe this is possible to do from a project report. Folders aren't referenceable from the project object to filter by.
An alternative approach you could take is to create a document report that is grouped by Project Name. From this type of report you can apply an exists filter that would only return documents that are in a folder with a specific name. In the example text mode below, we are only returning documents in folders called "Final Versions".
EXISTS:a:$$EXISTSMOD=EXISTS
EXISTS:a:$$OBJCODE=DOCFLD
EXISTS:a:documentID=FIELD:ID
EXISTS:a:folder:name=Final Versions
EXISTS:a:folder:name_Mod=eq
Best Regards,
Rich.