The sixth return of C_TransmogCollection.GetAppearanceSourceInfo (item link) returns the base version of the item for certain items. The appearance ID and transmog links seem to be correct.
For example, trying to link the mythic Stormbattered Casque from the wardrobe gives a link of the normal level 860 item.
http://www.wowhead.com/item=141424/s...=70&bonus=3466
The same seems to go for all artifact weapons, though they are not meant to be transmogged, I guess, so I can see why they would not be supported.
This affects linking items from the wardrobe, namely.