OK I chatted with Erin, he says Box3D doesn't have the ability to clone a world yet, but he does plan to add it. So maybe reach out to him. I think you'll need the clone a world functionality to do the rollback properly GGPO style.

Awesome, thanks for engaging with me and talking to Erin about it.

I hacked together world cloning in box 2d and am confident I could do it in box 3d as well. Especially with the record/replay/validate system to help guide the implementation.

I'll find a way to send you and Erin my project when I get it working (or when I fail)