Thanks for the reminder, @mflorea! Actually I also had this in mind but by the time I wrote this proposal I forgot about it , thanks for reminding!
Indeed. Actually, it’s rather easy to make a “deactivated” icon from an image icon by using a grayscale image filter (with some compatibility work), but this cannot be implemented as part of the icon theme API
As you said, the problem is that there is nothing that may look like an empty heart in Silk. My solution to that may turn towards dropping the support for the Silk icon set , I’ll keep you posted.
I’m gonna give it some more thought, to see whether I have some other idea, but if there are other ideas, I am taking