Artifically flavoured yogurt is allowed in phase 2 but yeah you're right, I don't think there is greek of this kind. What I do sometimes (because I do love plain greek yogurt without it being sweetened) is add a few drops of liquid stevia. You can also add some agave nectar yourself

I don't really use artificial sweeteners but if you're up for them, you add them to the yogurt yourself plus fruits and bam, SBD friendly sweet Greek yogurt