I've read others indicating that certain characters couldn't sign (including Tinkerbell, Pluto, and even Capt. Jack Sparrow), but we got autographs from all of them (though we did not encounter the ones you mention, and the Tink autograph took a little creative helping from CMs.)
If you have your heart set on a particular autograph, I would ask the CM assistant with the character. I've heard that they may be able to take your book 'backstage' for an autograph if you're willing to wait. I don't know if they can always do this, or for any character that can't sign, but I figure it's worth a shot if it's for a character you really want. Doesn't hurt to ask!
I thought I'd heard also that a non-signing character had a stamp, and I was actually expecting that's how Pluto would sign, would stamp a little pawprint or something, but he actually wrote his name! It would be really nice if non-signing characters were given some way (ie. a stamp) to place a memento in an autograph book.