I can't imagine how Lightroom would be able to do this unless there were metadata in the image files or distinguishing features in the filenames that could be used to determine which were monochrome. Probably the best bet would be to add a keyword to each monochrome image, then use a smart collection as a repository. Prospectively, that could be done at the time of import of a scan or monochrome capture, or conversion of a color original — and retrospectively by locating all the monochrome images visually and adding the keyword to them.
For an automated approach, it would be necessary to create a software application that was able to recurse its way through a hierarchy of image files and detect monochrome instances from innate characteristics of the files themselves; this would presumably require different tests for rendered and raw files. But building a robust tool would involve some heavy lifting and there still would be a secondary issue of how to flag the monochrome images so Lightroom could find them. Preferably without modifying the files themselves. Maybe a Lightroom plug-in?
