jason_iversenThe Maya USD plugin supports USD Preview Surface, RfM Shaders, Arnold Shaders, and RenderMan for Maya.
I'm assuming the plugin translates hypershade nodes into Mtlx networks?
Whereas, Arnold for Maya has its own USD export menu.
Since Arnold for Maya supports Material X, that might be an option. However, it means converting all the Arnold standard surfaces to Material X on the Maya side, then re-exporting to USD. Which is less than ideal.