It's a privlage to have a custom 3d item like the legends do. Even tho it's just a texture it still would be a new item type cuz you would need to create a new item on Toribash for that texture to be on that item cuz it's not possible right now to texture 3d items directly thro Toribash without creating a new item.
There was a similar thread a while ago, it was about recolors
http://forum.toribash.com/showthread.php?t=538091
TL;DR making textures for 3D objects is possible but really hard because they have different UV layout and stuff, letting people upload textures for 3D objects is gonna increase the storage usage and devs don't want that (we even had this incident recently when forums ran out of space and avatars weren't uploading).