Is there any particular reason this isn't possible? Such functionality would open up a number of interesting possibilities for me.
Animating opacity of attachments on the timeline
- Edited
You can animate the alpha and color of the slot containing your attachments, and it makes sense because if you want to switch image in between, that image will inherit the same alpha and color, making the animation seamless.
At runtime, the attachment color is part of the shared Skin and shared across all skeletons (even if it's the "default" skin) .
The slot color belongs to each individual skeleton.
You can animate the slot color. 
Oh my gosh, I didn't even think to try animating the slot. Wowww how am I this bad at trying things. 
Thank you!