You won't likely need highway buses, except for making the one-hour run between Yamaguchi and Hagi (knocking three hours off the train-travel time). Two companies operate bus routes: JR and Bocho bus lines. Japan Rail Passes are only valid for use on the JR buses.
Bocho Bus Center (防長バス). 防長バス 0834/32–7733; www.bochobus.co.jp.