So I tried first on my Titan, then my Hunter, and now finally on my Warlock, all to the same problem. Buying the Iron Pledge "Ornaments" does not unlock the items for use as ornaments, although the items would suggest so based on the item names and their iconography backgrounds (identical to that of an ornament).
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Saladin sells you the year 1 armor ornament variant. It can be freely applied to Iron Banner gear, but for non-IB gear you have to use Synthweave to unlock it.