I was asking myself exactly the same question (same type, same 180min ETOPS) and came to the conclusion there's no "software" way to do it, the only chance is to act as an aviator (and navigator). Initially turn back in HDG SEL and follow (roughly) 15NM offset on the reciprocal magnetic track. If time permits and no new clearance can be obtained, just insert the route in reverse and then you can set 15NM offset and enjoy the help of LNAV. It won't take too long as there are usually not too many waypoints on such routes.
Any other opinion is appreciated