It's working now.

After installing OSX from the USB drive a few times, each time noodling with how I was cloning the installation DVD to the USB drive, I concluded that the installation source wasn't the problem.

I booted into the installer and started Disk Utility and erased the SSD, writing zeros to the entire disk. I then re-installed the same way as I'd done several previous times, but this time it worked. I guess the installer wasn't overwriting whatever the problem was upon re-installation until I completely erased the drive. Whatever. As long as it works...