Several rows of 3x3 EC on the UD of the 744's would feel cramped, would be surprised if DL were to do a 3x2 or 2x3 ala MD8x/9x/717 for the UD. Storage space would be a challenge as others have mentioned for the UD as well.
However "what if" DL is thinking a 3x3 for the UD with slim seats at EC pitch (or a bit more) and for some missions/routes, do similar to FI/ICE where the center is blocked and sell a 2x2 at a premium above regular coach, yet below discounted J. Then for other routes/missions (e.g. beach/high-density, charters, etc) sell the full 3x3 config. This could even be done where part say first 5 rows of the UD is sold as an enhanced EC (e.g. same service, just more room) with the remaining rows being 3x3 EC.
Impossible? Lets way to see what DL will actually do...