I had root canals done when I was breastfeeding my first son. Because I had had SO MANY, I opted for the nitrous & pumped and dumped for the time period the surgeon said it would be in my system. I think the Novacaine, because it was local, was considered a non-issue, but I don't really remember now (and I was pumping/dumping anyway).
Talk to your oral surgeon/dentist about your specific concerns and put in a call to your local La Leche League as well. The doc wil likely consider the Hg (mercury) a non-issue, but if there is ANY concern, the LLL will know.