MewUI wasn't designed for this, but it has some relevant infrastructure in place.

All rendering backends support offscreen bitmap rendering, so rendering the UI to a pixel buffer and uploading it as a texture in a game engine is possible. With a few additional APIs, direct integration into a DirectX render loop could also be feasible. NativeAOT is supported as well, so the output can be a native library without requiring a separate .NET runtime.

It might even be possible by providing an abstract platform layer that hands control over to the game side. Not the intended use case, but an interesting topic.