I wanted to follow up with this now that UE5.6 is out. I have a USD stage loaded with USD-encoded MaterialX materials. This same USD file opens correctly in both Maya and Houdini, but not in Unreal. That is, it opens with all grey materials in UE.
I have the options purpose=render, and render context=mtlx. However, it looks like it is applying a USDPreviewSurfaceTwoSided. I have the plugins InterchangeOpenUSD, USDImporter, and USDcore loaded.
Any help in getting this to work would be greatly appreciated!