From what I understand, when you import FBX, it gets converted into UE4’s internal format. So Blender > OpenGEX > FBX > UE4 doesn’t make sense. OpenGEX would be converted into UE4 internal format on import, just like FBX does. So Blender > OpenGEX > UE4 is how it should be. Ditching FBX altogether. And that’s what UE4 plugin would do - read OpenGEX and convert it into internal format. I might be wrong though.
Might as well make a standalone app that takes OpenGEX and converts it into FBX. This way the converter app can closed source and using FBX SDK.