Is it possible for those attempting the Vagrant challenge to acquire the transmog cosmetic for the items they pick up along their journey, so when they do hit 60 they will have all the items they've gathered along the way? I would suggest that they can equip the item however it will provide zero stats, including armor so as it provides no benefit but they are otherwise able to learn the transmog.
This can incentivize people to do this challenge with the slow and steady one as once you hit 60 there would be a severe lack of transmog options so there almost seems no point in being rewarded all of those fashion coins.