"Is it possible to transfer a Maya (or Blender) rig directly into Unreal using FBX and keep the weighting?" That's kindof my point : by tackling a tiny portion first and taking it all the way down to the finish line (instead of doing a full character), you'll force yourself to run into this kind of question earlier than…
It's a case of "it depends". Here on UE5 Manny (top), the transition from soft parts to hard panels is continious, achieved with a very precise placement of loops perfectly following the transition (and therefore also dictating the ID/texturing mask, which ends up being nearly per-polygon hence very clean and robust). Yet…