I will attempt the same thing this Spring but I will use royal icing, like making the tiara, and try to form spikes as thorns. It may need to be stiffer than the crown icing to stand.
I think this will be not impossible but less successful. I think there will be a lot of breakage if you use royal icing, and if you plan to make it stiffer, then I would think you lose the "sharp" quality of the thorns therefore losing the significance of the crown itself. The Crown of Thorns has thorns on the bottom and top ...the bottom ones would have pierced the brow of Jesus so I think that one would want them to look sharply pointed which would be easier achieved by using gumpaste. Please know, this is just my humble opinion, and certainly there is no right or wrong way of doing it.