In fact it's not totally a custom skeleton.
It's exactly the same as devs from crytek provide back in time. I've just posed this biped rig slightly differently, in figure mode ( I say it a lot of time cause this is important), to match my silhouette. It's exported as an other skeleton, yes, but the only difference is the bones transform, normally handled by this figure feature if I don't misunderstood.
If you have time to read, the techart named CryDanny, which provide us Cemaxutils, was trying to help someone which was in close situation
https://www.cryengine.com/community_arc ... 7&start=15