Not so great update here.
By 5 months old, DS seemed much less happy and had dropped multiple major percentile lines on the growth chart. My milk supply was low no matter what I tried, even going off the diet completely. One side was marginal, and the other side wasn't producing anything at all. (Interestingly enough, that was the same side that my previous baby refused to nurse from after her heart surgery. It was producing plenty of milk back then, but she had a stiff neck, and wouldn't turn her head that way. Maybe it got tired of waiting, and decided to take early retirement?
So I started supplementing with formula, and he's been doing great since then, growing well and VERY cheerful and active. He's also grown a gorgeous head of hair, which I find interesting, as my other children had almost no hair at his age. Maybe my milk is low in Pantene Pro-Vitamins.
But still... argh! This is definitely not what we had in mind.
The diet was very likely a factor in the low milk supply, but probably not the only one. In hindsight, he wasn't a demanding newborn at all, and due to family events there were lots of people around to cuddle and play with him, so maybe I wasn't nursing him often enough in the beginning. This did cross my mind back then, but I didn't think much of it, as I've always tended toward major oversupply in the past (even with DD2, when I was near-exclusively pumping for the first two months).
In addition, DS was born with a tongue tie; it was clipped @ 1 week old, but it still looks a bit peculiar to me. When I started pumping to try and increase supply, I realized that the pump felt much more "natural" and effective than his nursing did. Unfortunately, by that point it seemed to be too late to make much difference in the situation.
And then there was the high-dose Vitamin D and extra calcium that I started taking. I thought it might increase my supply, but it actually seemed to decrease it even further. Maybe I'm imagining that, as I can't find any references that would support it, but it did seem that way.
On the other hand, Kwasniewski does say that "Optimal" children will be smaller, so maybe DS's size was really okay, and I should have just supplemented with some vegetable soup with butter (which the book recommends starting at around 4 months, IIRC). Without a community of experienced parents and doctors, though, I didn't feel comfortable making that judgment. He really did seem to want much more food than he was getting.
I really wish I knew the full story -- not just from JK's books -- so I could decide whether or not to keep my family on the diet. Other than the milk supply issues, everything has gone great... better general health for me, a much easier and happier pregnancy than previous times, and a baby who's bright, cheerful, and easy-going. But I'm not sure what to do now.