Ok, update.
Alignment at the dealer today revealed a problem. Everything is fine except for the front camber: the bushings on the lower control arms are seized up and will barely move. While things were fine with my lowering springs, the best tech could get out at stock height was slight positive camber on the left, and slight negative on the right. And he had to impact hammer it pretty darn good to get there. The reason this wasn't a problem during the last alignment is that the cams had just enough adjustment to work with the lowering to get the settings I wanted.
Apparently they don't use anti-seize at the factory in the camber cams, and so what happens is that the sleeve inside the bushing that contains the camber bolt seizes up. The problem is that you can't just replace the bushings, you have to replace the entire lower control arm. Which is $550 in parts (MSRP), plus 5 hours labor, so I'm looking at a nice $1000 repair bill to do this. I'm not confident I can do the labor myself, because getting the existing cam bolt will require torching, hammering, etc (which will ruin the bushing, which is why the LCA needs to be replaced), and I don't think I have the tools to get the job done. The tech said that they usually replace these under warranty, but I'm 3.7K over warranty. Once the LCAs are replaced, I'll be able to do all the alignment adjustments I ever want again. I drove 2.5 years before this happened. I don't know if the frequent alignments exacerbated the problem (I think the opposite would be the case).
The tech said that he puts on antiseize on the cam bolts, so that this problem doesn't occur again (or at least not for a much longer time). The camber adjustments bolts are right there, in the front path of the salt spray from the roads, so I can definitely see how the New England driving conditions accelerate the seizing.
SO. I would suggest that any RX-8 owners (and probably NC Miata owners too, since those have a similar suspension design) that drive the car in the winter replace the camber cams before they seize up, and put antiseize on them. For a brand new car, you can probably just take the bolt out, put antiseize on it and put the bolt back in, but for a car with some miles on it, it's worth replacing the cam bolt (according to the tech). When replacing the cam, the tech suggested using some sandpaper or wire brush to get any rusty spots out and smooth out the sleeve.
Holy s*** though, that's an expensive repair bill.