Halfway, but I could foresee using a sort of reverse image version of it on the project I just started. I am talking about how you extended icon-heart to icon-heart-empty.
I guess you would take the little "white" mini drop/glare inside and turn it black, then remove the fill and just keep the outer drop outline. If it looks okay and isn't too much trouble, I would probably put it to use. I do have an old version of some font software I might be able to figure it out if you end up sharing files too.
Nice stuff, going to try to use it sometime!