Using spine, attaching skin placeholders from other skins.

SkinChanger.cs

using Spine.Unity;
using UnityEngine;
using System.Collections.Generic;

[RequireComponent(typeof(SkeletonAnimation))]
public class SkinChanger : MonoBehaviour {

	private SkeletonAnimation sa;
	private Skin originalSkin;

	void Start () {
		sa = GetComponent<SkeletonAnimation> ();
		originalSkin = sa.skeleton.Skin;
	}

	public void SkinSwap(string skinName, string placeholderName)
	{
		Skin tmpSkin = sa.skeleton.data.FindSkin (skinName);
		Skin combinedSkin = new Skin ("Combined");

		foreach (KeyValuePair<Spine.Skin.AttachmentKeyTuple,Spine.Attachment> a in originalSkin.Attachments) {
			int slotIndex = a.Key.slotIndex;
			string attachmentName = a.Key.name;
			Attachment attachment = a.Value;
			combinedSkin.AddAttachment (slotIndex, attachmentName, attachment);
		}

		foreach (KeyValuePair<Spine.Skin.AttachmentKeyTuple,Spine.Attachment> a in tmpSkin.Attachments) {
			int slotIndex = a.Key.slotIndex;
			string attachmentName = a.Key.name;
			Attachment attachment = a.Value;
			if (attachmentName == placeholderName) {
				combinedSkin.AddAttachment (slotIndex, attachmentName, attachment);
			}
		}

		sa.skeleton.SetSkin (combinedSkin);
		sa.skeleton.SetSlotsToSetupPose ();
	}
}

WeaponSwap.cs

// This is just a test using the goblin project and setting the spear as a weapon for one skin, using skin placeholders and a dagger for the other skin.

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(SkinChanger))]
public class WeaponSwap : MonoBehaviour {

	private SkinChanger sc;
	private bool isDagger = true;

	// Use this for initialization
	private void Start () {
		sc = GetComponent<SkinChanger> ();
	}
	
	public void OnButtonClick()
	{
		isDagger = !isDagger;
		if (isDagger) {
			sc.SkinSwap ("goblingirl", "weapon");
		} else {
			sc.SkinSwap ("goblin", "weapon");
		}
	}
}