Luckily for me, one of the testers for this pattern (called, appropriately, "Riblet") is someone who knows virtually everything about crochet. I'm expecting that Donna might even tell me if this stitch already has a name.
For the time being, I'm calling it "Upper Front Post Half Double Crochet" and "Upper Back Post Half Double Crochet." Or U-FPhdc and U-BPhdc, for short.
Here's a diagram of a U-FPhdc as it's worked:
The reverse side of the ribbing, which looks slightly different, is pretty, as well.