i have a minor cavity and my dentist wants me to take a prescription fluoride to brush w...stays on your teeth overnight and fill the cavity. I'm personally not down with either b/c still bf'ing and trying for number two.
How old is your baby? Is it the baby in your avatar? I wouldn't be worried taking almost anything with a baby that age, though for medications I'd check with Hale to make sure something was OK. But most things are. I've had oral surgery while breastfeeding a pretty young baby. You don't have to be nearly as careful when you're breastfeeding as pregnant. Very few things get in the breastmilk at high enough quantaties to do anything.
I'd be more concerned about what the filling was going to be made out of and avoid mercury fillings, but personally I'd want to get it filled while it's small. You should continue to take care of yourself, including dental health, while breastfeeding. Women with several children might go a decade or more with continuous or near continuous pregnancy and breastfeeding. You can't put off your own care for that long. That isn't good for you or your kids, and bad dental health stresses your system which isn't good for pregnancy either.
I will bump this too to see if I'm a minority viewpoint here. But good luck no matter what you choose!
Personally I would not recommend it if you are breast feeding. Chances are you will ingest some of it while sleeping. Fluorosis is a strong possibility if the fluoride is passed on to your child. Moreso if you use other fluoridated sources like tap water etc...