You can pose the character manually and pose the metarig using the UE4 Mannequin as a target. This will fix the metarig and add twist/other bones.
After that generate the rig and parent the character to the rig as usual. “Build Unreal Skeleton” does not rely on anything from the target character. So the rig will still work and you can still animate and export to Unreal Engine.
Only limitation is the posing functionality for the character is not available.