If there will be this type of implementation I would say; through BIOS update or different NUC revision.
I do not have official notifications about this matter.
My older Sandy Bridge laptop can run 2133MHz memory so I don't believe this is a hardware limitation - surely Broadwell's IMC is much more advanced? Faster RAM may not make a lot of difference with processor specific tasks but in my experience it greatly helps Intel's integrated graphics in games etc.
It would be great if this could be implemented further down the road.