You can train and train, but you will have to decide (or get some input from your girl's breeder) about the judges you enter her under.
Are they more concerned with the dog being super trained and looking straight-forward at all times, or the tailset.
I can tell you as a judge, I'd like to be able to stand in the rind and see all the dog's profiles. So, for me, I'd like to see the dog with it's head facing forward as I scan the ring. It won't matter if you hold her tail down on the back (ie., lowset tails) as I would notice it when I went over the dog with my hands. It isn't something that will be missed.
If your girl is young, I'd be far more concerned with her behaving exactly as you want her to (stacking and facing forward),that way as she ages, she will get that down pat and then you won't have to worry about it and you can hold her tail as you want it to be presented. You don't want to be tugging on the lead giving her a non-happy experience in the ring at a young age… basenjis are impressionable and won't forget if they get in trouble in the ring!!
Good luck to her and you both!