oh that leads me to another potential solution: making your own twist bones.
for that remove the twist bones from the Biped completely, add some extra non-biped bones for twists, parent them to the Biped matching the hierarchy of the UE4 skeleton, and then add some special rigging controllers in Max to make them behave like twist bones. then re-weight the mesh to the skeleton
I’m not versed enough in rigging controllers in Max to know if it would work, but another solution might be to add some extra non-biped bones for twists, parent them to match the UE4 skeleton, and then add some controllers so they follow the original Biped twist bones. then in the mesh remove the weights of the original Biped twists and add the new ones
well that’s some room for experimentation. it’s a shame these semi-exotic solutions are needed in the first place, but at least it would be a one-time-setup workaround