There's no connection between OC and SD in 48 unfortunately.
Not defending ERM here, but there is a road from OC to the north side of Pendleton. The military owns all the land, but only perminantly resides in a small fraction - the rest is nature preserves sometimes used for various exercises. Basilone Road goes from south of San Clemente into the settlement proper, and then ammunition road takes you to Fallbrook. Of course it's not year round since the military may close the base to civilians at specific moments, terminating the connection.
I think Temecula in Riverside and the surroundings are more connected to San Diego than going along the coast. It would result in splitting Riverside, but that SW Riverside County area is more connected to San Diego County in some ways. Another option would be to just squeeze the coastal district more northward so that the Eastern district can start to take up more of the SD County.
