If you can to browse the exact point where you leave the road you could send the ULR which contains the co ords in it like this http://www.openstreetmap.org/#map=16/51.7268/-1.9324 you could also send the campsite location like this http://www.openstreetmap.org/#map=16/51.7311/-1.9300 Satnavs do have the option to enter lat longs if there isn't a postcode or address. Using the share button on the slippy map allows you to add a marker to the map. If you record your new road as a GPX with a stanav satnav or a gps you can upload the add trace to OSM. OSM ready for mapping. If you can map it from the licensed aerial that is available as a background in iD Potlatch2 or JOSM editors it can be mapped from local knowledge. knowledge quite quickly. If the route from the road is long and confusing you could, if you don't have time to map new road then send them a series for co ords to follow ( known as a route) assuming they can use them in their satnav or gps.
