I was working on my mansion on maids server and i was making a new room which is void themed. I had other ops coming by every now and then asking who greifed it. So i think i have heard talk of this, but ; we can use signs in SP. Why not in MP? The way i see this working is:
You make blocks in paint like you would normally, just individually. Then upload it to your prefrences, then when you are in multi player you can place them. Now i see this being abused so maybe you can have a max of 5 custom blocks at a time, and only 4 of each can be placed on a server. now why did i bring up my building? Well if i made a sign (block), uploaded it, and placed it in that room and it would read "VOID ROOM" so people would not try and rebuild it or find the person who did it. Aside from this is see many other uses for this.