I will look into this there is a plugin that allows you to turn into mobs and animals. I would assume that it is possible to do the same with blocks. As said, I will look into it. EDIT by Moderator: merged posts, please use the edit button instead of double posting.
Err, I don't think turning into a block is the same MobDisquise uses packets to make the player look like a *entity*. I guess we could ask @desmin88