UV uses a middleware called Awesomium which is a client-server wrapper around Chrome, probably a version with extensive crypto re-writes (yes, even the plugin uses it). As such, I believe the only way to do letterbox properly on a 21:9 screen or projector is by using Ctrl + Mouse wheel up to scale up. That's about it. But that might stretch the horizontal beyond the limits.
I tried Flixster once, and it was in HD.
I tried Flixster once, and it was in HD.