I know this can be done easily with NBTedit or SethBling's filter for MCEdit, but I would love to be able to do this in vanilla. Or is this going to be like player heads and remain accessible only through an external editor?
I think this would be a nice feature for map makers, but honestly I don't think it's going to happen. And because with Sethbling's MCedit filter, it's really easy to use. I'd now, because I have the filter. This is actually something I've never thought about before! Thanks for bringing it up!