Sorry i didnt see this earlier too!
Asa dental assistant for the past 13 years and someone who had a HEAP of dental work done while pregnant and while BF I just wanted to say that you will be fine to feed, the anaesthetics used in dental work are really fast acting and also fast to wear off these days (heaps easier for the body to metabolise) but as was just said, if you are really worried about it try to feed/express before the treatment and then again afterwards and had some EBM to bottle feed with.

Good luck1!