Otherwise known as Stagecraft. Not sure if 1899 was filmed on one of those stages. According to Wikipedia, here are the current locations:
Note that ILM can "pop-up" new stages. Not sure what this means as I would imagine just getting all the material (screens, electronics and rigging to hold everything in place) is not a trivial task.
Also, there is more than just Unreal driving those LED screens. ILM actually partnered with Epic to develop a proprietary set of tools for Stagecraft (it has its own brand, but I can't remember what it was called). I am also led to believe that future iterations helped solve the problem of getting the screens to blend better with the floor and (more importantly) the ceiling LED's. Now the camera can pan up with fewer restrictions.
Interesting bit of history: It probably took some closer collaboration between ILM and Epic to get this done and every time two companies need to work that close together, there are always bumps and rough edges. Well, I would imagine that Epic's CTO, Kim Libreri, was instrumental in making that happen. Kim used to be a visual effects supervisor at ILM, leaving ILM and joining Epic in 2014. Small world coincidence? I would not be surprised if Kim's move to Epic was engineered just for things like Stagecraft. Afterall, George Lucas has wanted to do a more virtual world of production for something like this for over 20 years. The only way to get there is through game technology.
Dave