My apologies, I thought you asking for an example meant a clarification on what he was attempting to do. As for usefulness, I could not say. I personally only see a limited use case for it, because: Head textures would have to be specifically designed to use this new method of decoration, meaning very few people would actually be able to use this change to its full potential. Current head textures are already quite creative, and tbh I am not sure where this would allow for more creativity. That is something a deco team member or somebody else who works with heads/skin textures might be able to answer more sufficiently. I see where there is a potential for new head designs and creative options, though whether that potential would actually be worth the time it would take to code and apply any necessary changes to EMC, I am not sure. Somebody more creative than me, however, might be able to create something that would show the usefulness of this change.
I've seen other players using their heads to decorate their builds but never have used it myself since, if I understand it correctly, I would have to keep my skin the same once I used it in order to retain the block's appearance. Something I have wondered is whether it would be possible to save a copy of what the head looks like when I place it, store the skin on a server somewhere, then use that to skin the block later on rather than the player's skin. If it is even possible I think it would be an attractive feature. Possibly an interface could be devised to select a skin from several stored ones. A base number of stored skins would be allowed to regular users then additional ones based on Supporter level. I've also wished there were a way to make custom paintings. Even if it were a bit tedious like making custom maps I think a lot of people would find creative ways to use them. Banner designs were a very limited step in this direction.
In the current system each head retains the skin file from when the player was killed. If you change the skin file, then new heads will have new skin, but the old ones will keep the old file.
