Hey Fabrice,
JedTheKrampus has beaten me to it and got the reference implementation of MikkTSpace into UE4. His pull request is over here if you want to give it a go locally; https://github.com/EpicGames/UnrealEngine/pull/561
I’ve been testing it and it works fine - although you’ll still need to bake normal maps from xNormal with +X-Y+Z, as before.