I duplicate the project, change the new projects settings to SD and export a master file. Then bring that SD .mov into Compressor, apply the Apple DVD Disc burning settings and set job action to create DVD. I have had a problem with the text in titles not scaling down when exporting from the duplicated SD Project. Copy/pasting the title from the HD Project back into the SD Project fixes it. Or avoid the text issue by first exporting the HD Project, then bring that file into an SD Project and export again. Either way, I found this method to be comparable to either Matrox MXO2 or AJA KiPro hardware scaling.
There’s also the Share> DVD function right in FCPX, but I wasn’t happy with the results from an HD project. Honestly I can’t recall whether I tested Share> DVD from a duplicated HD project changed to SD.