I don't see the need for such a folder which would defeat the main purpose of libraries: code reuse by different projects. If a library is just for a single project, then it shouldn't be compiled into a library in the first place; its source code should be added to the project instead.
For example, if you have modified a b4xlib file only for a single project, it's better to add directly the modified source code to the project than have a modified library in the proposed project-specific library folder.