Here's a big pic of the event run if it helps any. I probably have a track along the gravel roads but don't see one immediately-
http://130.94.161.162/Maps/Oregon/So...gon/McGrew.jpg
I am pretty sure the way we've always gone out is at the saddle closer to the campground (says "Cleopatra Mine" on my map).. follow that road (labelled 302) back to the heavy dash-lined road (112 / 4402) and on out to O'Brien. This is to figure how to get into the campground directly if it is being run in reverse.
I have a number of TOPO! files with various McGrew names on them you can load and see if you get anything reasonable. Can't vouch for what's in them. I bet one is some planning of routes out of Cave Junction to the coast.
(first one is probably best, has tracks in and out)
http://130.94.161.162/Maps/Oregon/So...gon_tracks.tpo
http://130.94.161.162/Maps/Oregon/So...20planning.tpo
http://130.94.161.162/Maps/Oregon/So...ew%20Track.tpo