Well, this is a toughy. First of all, I agree with everyone else. It is your day and you certainly can't have every person who wants to be in the bridal party, IN the bridal party. Most people understand this, to others it means that you don't care about them. To that I say, whatever. Because this woman is clearly a little egocentric, I would probably ask someone who knows her well how she will take being asked to do another job. She may be offended, or feel that you pity her by giving her "guestbook duty". I don't know her at all, but I think she wants to be standing up there with the bridal party, included in the pics, center of attention, etc. Giving her other wedding day jobs won't cut it. I don't think she really wants to "help", she just wants to be included. Therefore before you ask her, ask someone else if she will consider it. Otherwise she may just get more upset.
That's just my 2 cents though!