Almost have this working... The code is very similar to what I've done in spine_c and rust.

This is a test:

            var skeletonAsset = handle.Result;
            var skeletonAnimation = SkeletonAnimation.AddToGameObject(characterGO, skeletonAsset);
            skeletonAnimation.Initialize(true);
            // skeletonAnimation.AnimationState.SetAnimation(0, "idle", true);

            var attachmentRegion = skeletonAsset.atlasAssets[0].GetAtlas().FindRegion("human_body_01");
            Assert.IsNotNull(attachmentRegion);
            Debug.Log("region: " + attachmentRegion.name);

            var regionAttachment = new RegionAttachment("human_body_01")
            {
                Region = attachmentRegion,
                Width = 1f,
                Height = 1f,
                X = 0f,
                Y = 0f,
                ScaleX = 1f,
                ScaleY = 1f,
                Rotation = 0f
            };
            regionAttachment.UpdateRegion();

            var skeleton = skeletonAnimation.Skeleton;
            var slot = skeleton.FindSlot("body");
            Assert.IsNotNull(slot);
            var slotIndex = slot.Data.Index;
            Debug.Log("Body slot index: " + slotIndex);
            var characterSkin = new Skin("character-base");
            characterSkin.SetAttachment(slotIndex, "human_body_01", regionAttachment);
            skeleton.SetSkin(characterSkin);
            skeleton.SetSlotsToSetupPose();
            skeletonAnimation.AnimationState.Apply(skeleton);
            skeleton.UpdateWorldTransform(Skeleton.Physics.Update);

There are no errors, but the new region attachment doesn't appear. I know the character is there due to a reference region attachment that's exported which shows up.

Is there a step I'm missing? Maybe re-generating the skeleton data in some way?