I had a similar problem with something called the 'Vir Banal'ras' I think it was. Hint might be in the name, who knows? It's a rogue-only, elf-trained only armour.
While I agree that it seems sort of odd to have a piece drop that no one can wear (my Inquisitor was neither elf nor rogue), wasn't there a lot of emphasis on your companions having their own 'individual style' and/or 'personal aesthetic?' So there's just no way that Sera would be caught dead wearing anything remotely 'elfy,' is there? Quite frankly, I find it somewhat creepy that she spends so much time being all 'boo, elfy elves are the worst!' but she keeps making these weird backhanded 'you're really attractive' comments about my Dalish elf that I'm sure she never said to Trevelyan. Then again, Sera and Vivienne are two I haven't really bothered with trying to bond with yet so there's probably something going on there that's just over my head right now. Or...y'know...this is a bug and I'm being 'too forgiving' or something. xD
The mage armour that is elf-trained, mage-only, Solas could wear. But he's a walking jumble of contradictions at the best of times.