I would be concerned about the balloon bursting, and not all of the pieces being removed. It could lead to infection or some such.

Would an enema work up front as well as anally? I don't have any experience with them personally. A large dildo, or some similar object, could hold it in for a bit at least...would probably require a good deal of effort and clenching to keep it all in, too.

I -love- the embarrassment idea. The question, though, is would it be better (ie, crueler) to tell her what was going to happen, and let her dread it or even do it herself, or make it a surprise?