Yes, it is usually up to the person who is putting it on. I was lucky when Chuck Liddell did a book signing. The book store that was putting it on was nice enough to allow everyone who purchased a book, to get one piece of memorabilia signed.
If it is a problem, does anyone think it is better to be at the back of the line? After they sign your book, would it be more likely to have the player sign a ball since there wouldn't be many people behind you? If I have a chance to try this sometime, I will let you know how it goes.