My usual play for transcon is to waitlist for mileage upgrades on both segments at time of ticketing, then cancel the return request if the outbound clears, or take my chances if I ride in Y on the way out. With all applicable bonuses, a EWR-LAX trip will get me about 11,500 RDM, so a 3500-mile net loss is really worth it to me to secure F on at least one leg if I'm lucky.

I've had reasonably good success with this strategy in the past, and it works for me since I don't do transcons *that* often.