I figured out the problem. My previous project had been HD, and I must have imported assets before switching preferences back to SD first. Therefore, although I had selected SD, it only applies to new projects. After saving the templates, I started over, switched preferences first to SD, then re-imported all assets and reauthored the disc. This time it produced a standard .img with a VIDEO_TS file. Burning a with Toast produces a functional disc. Problem solved.
Thanks for the suggestion.