As posted by a user on the WoW subreddit who collected enough bones on his cloth wearer,
He was under the impression from what most people said before the event, that you could acquire the cosmetics on any class, and therefore it would not matter / you wouldnt be forced to roll a certain class.
For all other cosmetics that seems to be true, except for Tusks of Mannoroth,
They are currently unable to be used on any class other than a plate wearer.
He posted an image of him attempting to use them to show proof, but I cant seem to post the image here.
Also, I believe that Timerunners inventories are wiped after the event, so I don’t believe you can keep them in your inventory until TWW.
Hope this gets changed for the people who want the cosmetic, and I’m sure a lot of people are currently farming bones on classes that are not eligible to use the Tusks.