It would be nice if there were some system of being able to get a specific piece if you really want it. My thought would just be to add the drops to a vendor for purchase with Poetics. Require that you've actually completed the dungeon itself and maybe lock the vendor out until after that current patch (so 5.1's dungeon gear wouldn't be purchaseable until patch 5.2 goes live for example). Also maybe even make the Poetic cost fairly high relative to buying actual upgrade gear at the time, like 1000 Poetics for a chest piece when you can buy an actual gear upgrade for 800 or whatever it is.

I'm assuming part of their design intention is to keep more people running a variety of dungeons such as to fill out DF groups so they would likely want some level of friction that would make "just running the dungeon" the more appealing route. This would help for those occasions where you just want one specific piece and/or are just having terrible drop luck.