One the qualities of what you are describing is that the overlay networks do not have to be large. Almost every "LAN over IP" project you will find on the internet tries to be able to scale to infinity, so most of them are not appropriate for what you describe. However there is at least one I know of that does not try to scale like that and it works well. It is easy to segregate "LANs" (family, friends, colleagues). Each one can have its own TAP interface.