Hey,
People have probably found this before, but I discovered a way to locate possible footpaths in urban areas online using Google Maps. When you go to an urban area in Google Maps in "map" mode (not satellite mode), and zoom in, it shows you the boundaries of each property. And if you look in a hilly area (e.g. Berkeley hills), every once in a while you will see an interesting "gap" (set of very closely spaced parallel lines) that streches from one street to another. This means that it is a thin strip of public land. This is a good indication of a pedestrian path. Sometimes it is not; it could be just trees or something; but more often than not it will be either a paved path, or at least a grassy passable area. Of course, you should double-check with satellite mode, Google street view, or by visiting in person. I've added several of these footpaths onto OpenStreetMap (some I'm sure are real based on street view; some very unsure). I figure, if I'm wrong, what's the worst that'll happen? (Someone will go there, find it wrong, and then delete it.) But for the ones that are right, I think it is very valuable information for the public.