Until GW officially releases a model, that should not be able to restrict them. For the above, I would use just the wolf, green-stuff his fur a bit more shaggy, and then use a SW rider.You're not allowed to use these models to represent Thunderwolves in Games Workshop - private gaming only.
I had 5 TWC made up using Space Wolf riders on the backs of Laris and War Wolves from the CoO army from the other wargame, really nice models as they're about half the height of Fangir and also come with fairly celtic armour.
I brought them down to play at GW and was told to take them off the table. When I retorted that GW did not produce TWC models, but had them in an army list, so how can they refuse me the right to play with custom-made models I was simply told to take them off the table or leave.
They did tell me that plastic Thunderwolves should be getting released with the second wave of Space Wolves minature releases