Is that Aladdin/Jafar clip actually part of the show? Is that known?
It was confirmed that it’s the opening scene of the premiere, actually:
At the close of Once‘s panel, cocreators Adam Horowitz and Eddy Kitsis gave the audience one more treat — in the form of the first three minutes from the Sunday, Sept. 25 season premiere.
Source: http://tvline.com/2016/07/23/once-upon-a-time-season-6-aladdin-jafar-cast-oden-fehr/
I even recall Adam and Eddy specifying that, in one of the filmed videos of the panel.
They have shown real episode clips at San Diego Comic Con before, actually. At Comic Con 2014, they showed two clips from 401: one clip was the scene of Anna and Elsa at their parents grave, while the other was Regina releasing Sidney from his cell. The reason they don’t normally get to show actual episode clips at Comic Con, is because normally they only started filming a few days before. Comic Con was at a later date this year and in 2014, so they had actual episode footage ready to show.