To simplify the issue, in maya I used a simple cube, which I skinned to the whole Metahuman joint hierarchy, and paint weighted to the cube mesh to the “FACIAL_C_jaw” joint of the MetaHuman rig.
Selected the Cube only and exported it out via fbx.
imported it into it into UE 5.6.1, and in the import pop-up settings under “Common skeletal Meshes and Animations”, I’ve tried adding the “Face_Archtype_skeleton”, “metahuman_base_skel” as well as leaving it at “None”.
in UE, in the metahuman character blueprint component section, I’ve tried adding the cube under the “Body” as well as the “Face” like you suggested, with all the 3 different skeletal mesh options mentioned in section 2. above.