Thanks for the feedback! And glad to hear it's still useful. I wasn't sure how things would shake out with the new transmog feature.
I've not thought too much about how best to integrate with the new transmog. It appears that there are 2 seperate states for a tabard regarding transmog and achievements. Those states are "collected" and "equipped". Adding a new tabard to your inventory makes it available via transmog - "collected". But you still have to equip it to make it count for the achievements - "equipped". At this time the addon only cares about the achievements and if the tabard has been equipped.
Can you give me more detail of your idea to help me understand? Are you wanting the addon to be aware of the "collected" vs "equipped" status? It may be best to keep them seperate via a new tab along the bottom or something to avoid confusion.
FYI - This is just brainstorming mode. I've no idea what is actually possible to do yet.