honestly, the best way to deal with delta's deliberate failure to process upgrades for empty seats in domestic F is to register a complaint every single time you see it. there's a decent chance they will throw some miles at you as a conciliatory gesture.
if everyone does this, delta may finally realize that it's more cost-effective to fix their upgrade policy than to allow GAs the discretion to skip processing upgrades or to decide for themselves whether to upgrade passengers from coach with the "secret"/official upgrade list (the one that does not remove your place on the list when you board).