Obviously we live in a culture that finds breastfeeding beyond a year to be an oddity. Are there psychological consequences for the mother or child to consider when "going against society's grain" in such a way?

Not that I am aware of. Children (and even adults) tend to think that the way they live is "normal and natural and what everybody does." My youngest Alex knows lots of children who nurse as long as he did, or longer, and just assumes that all the kids in his daycare/kindergarten are nursing to sleep at night. He is aware that some babies aren't lucky, and don't get to nurse at all. He is aware that advertisers use breasts to sell beer and lingerie and other items.

I would say that the "cognitive dissonance" children might experience when they find out that not everyone gets to nurse as long as they did is similar to that they experience when they first run across racism or sexism or classism or religious intolerance, if they've been raised to be non-racist, non-sexist, non-classist, and religiously tolerant. The fact that we live in a racist society does not mean that I must give in to it and teach my children that racism is fine. The fact that we live in a sexist society does not mean that I must give in to it and teach my children that sexism is fine. And so on. I firmly believe in taking a stance for what I believe to be right. I teach my children to not be racist, not be sexist, not be classist, to respect everyone's religious beliefs (even those who don't respect ours), and so on. I teach them that breastfeeding until six to seven years is normal and natural, and to pity those children who never get to breastfeed.