Color me also confused by that first post!
My understanding is that characters will sign just about anything that's not a.) inappropriate to bring into the parks in the first place or b.) part of or being worn by a guest (soo...no skin, no t-shirt you're currently wearing, etc.). I had a ton of characters sign a tote bag with no problem, for instance; just be sure to bring a marker that works well on the surface you want them to sign.