While it is true you can only have the 12 fel glands, the same does not hold true for oter Netherwing quest items.  Items such as flayer hides and crystals can be farmed as often as you like.  Don't even have to have the quest to farm them and you can have more than the required number for the quest.  I make it a point to go there and skin carcasses left behind.  I haven't tried to see if the tabards will drop after you have already done the daily quest.