I would call and ask what they use to whiten them, as there are so many different methods and products. Then you can do some research and see what's safest. If it's a one time thing, it should be relatively safe. You can always pump and dump and feed expressed milk for a short period. But really, nothing much stays in breast milk for very long, so you could probably work around it.
Honestly though, you might find the last thing you are worrying about once you have a baby is how white your teeth are. It took me 6 months to even manage getting a hair cut, and in the first two months, I actually had to write myself a daily reminder to even brush my teeth! You're very much in auto-pilot just trying to get through the day, especially as you adjust to BF.