I hope that you are deadlifting significantly over 600lbs because if not, and you are worrying about such a trivial thing, then you've got your priorities seriously messed up.
Do you think that the strongest deadlifter freak out about "finding a movement to train the lower back dynamically"? Heck no! 95% of the strongest powerlifters and olympic lifters do not do that type of targeted work and those that do (there are a few olympic lifters that do some stiff leg rounded back lifting, but it is a huge minority) do not do it to a significant extent.
Yet these guys have the strongest lower backs in the world. And if you tested them "dynamically" they would score through the roof.
Deadlift/pull big weights with good form, perform the olympic lifts and squat heavy and stop majoring in the minor.
NOTE: Some olympic lifters (Klokov and some chinese lifters) do a very small amount of rounded back lifting, but these guys are already at the top of their physical capacities and investing in something that might help them by 0.5 - 1% might be worth the time and energy. But for someone who is not anywhere near that level, the efforts and adaptation energy would be better spent elsewhere.
But if you have some weird mental issue about training the lower back dynamically, find a place where strongmen train and do the Atlas stones.