I'm going to that event also. Can't you just put her in her carrier? Some of them (like my Celltei carrier) have a little thing to put the seat belt through. I would definately contain her. If you have to step on the brakes suddenly for whatever reason she could go flying off the seat and get hurt. And she is, of course, distracting - what little, adorable Yorkie riding in a car with you wouldn't be distracting.
Oh, and don't put her in the front seat beside you if you have airbags. If the airbag deploys she could be injured or killed.