A bit of an aside: If the route is widely known and acknowledged to exist, but perhaps isn’t officially recognised, I still think it is fine to add it to OSM. A long distance path in Gloucestershire is currently being mapped (not by me) which is not an official LDP but it is certainly one I know about and obviously other contributors do as well.

A network tag (on route relations) allows people to style official routes differently.

Vclaw, that’s a nice detailed description of how to overlay custom route information in Kosmos. That’ll come in handy.