I love the recent addition that puts an icon next to a motif, recipe, or furnishing plan which indicates that the current character has not yet learned it! But I'd like a little more along these lines.

It's very common for many of us have a "master crafter" character, and that might not be the "main" character being played most of the time. We'll typically want to pass any unlearned items to that master crafter for it to learn through our bank or storage. But when we're on a different character, there's no way of knowing if our master crafter knows an item or not. As our master crafter gains a large knowledge base, it's increasingly frustrating to be constantly passing it items that it already knows. Worse yet, if an item is encountered for sale, we don't know if it's worth purchasing for our master crafter or if it's already known.

It would be very useful if the current functionality could be extended to reflect knowledge across all characters in an account.
Ideally a different icon could indicate that the item has been learned on some other character. Even better if a tooltip might tell us which characters know the item, perhaps when we hover over the icon.

It seems like an AddOn should be able to track this knowledge across an account (it might require initially logging into each character to build the knowledge base).
On searching the AddOns list, I don't see anything current that provides this functionality. I am a dev myself and would consider writing something like this, if none actually exists. Hopefully the SavedVars content wouldn't become overly large. A compact representation shouldn't take more than a few bytes per known item.

My questions for the community:
Does an AddOn like this already exist?
Do we believe ZOS is already working on this?
Does this sound like a viable functionality for an AddOn to implement? What would be the technical challenges?