Seeing as how it is so difficult to find unbiased studies, let alone unbiased studies on BF and pot (neither one makes any $ for anyone willing to spend $ on research) we'll probably never know if smoking pot during BF is harmful, neutral or even beneficial (wouldn't that be great!!).
I've done a lot of BFing in my day, and researched it quite a bit too, and what I've learned about alcohol and BFing is that it takes roughly a six-pack a day for a week for enough alcohol to get into your milk to affect your baby, and that was a conservative estimate. I'd be more concerned about a mother's ability to care for her baby safely after consuming so much alcohol than the effect of the milk itself.
I would personally be concerned about smoking pot while PG just because of the fact that anytime you smoke anything you deprive your baby of oxygen (hence the tea/tincture/edible suggestion), but usually when you smoke pot a little goes a long way, so it's not too much of an issue. I also wouldn't smoke in front of my kids, but that's just me--It also bothers me when people talk excessively about booze, as in, "boy, we need some beer" or, "What you need is a drink" in front of them.
I have quite a bit of faith in the breast--it does a great job of filtering out impurities and harmful substances, and *probably* lets through "food substances" more readily, which is IMO why dairy may get through, causing allergies, but you can drink quite a bit of alcohol before enough will get through to affect your child.
All I know for sure is that after four years of nursing, (two years tandem) and (mostly) patiently caring for my kids all day, every day, cocktail hour with friends or the occasional pot induced giggle-fest does WONDERS for my own sense of self, which can get totally lost in the constant demands of my two beautiful, brilliant, BF'ing children. We do need to keep our children's needs first, but can't forget to let our hair down!