Maya USD Export - Arnold for Maya
With a version of Maya USD 0.15 or higher, an export option called Plug-in Configuration is available. Switching this option to Arnold will let mayaUSD export all of the necessary Arnold data on top of MayaUSD exports. It will then be possible to render the scene in standalone by running kick scene.usd. Also, loading this file with mayausd as a proxy shape or through an aiStandIn will allow to render it on top of another scene.
Known limitations:
Importing such a USD file to Maya won't import all the Arnold data back to Maya.
In order to kick the export USD file, it is necessary to create a camera so that MayaUSD exports it. The default "persp" camera will be ignored by mayaUSD.
Note that if you don't see the "Arnold" in the "Plug-in Configuration" list, it means that you don't have a version of MtoA that is compatible with the installed version of MayaUSD.