Custom shading mode help / issue with pxrRis shading mode.

[LPvfx]

Hi there,
I've been using the usd exporter to try and export the prman materials + geometry.
This works wonderfully for the renderman shaders that ship with renderman but if you have custom shaders installed it doesn't work.

Looking into it more closely the pxrRis shadingMode for the exporter :
https://github.com/Autodesk/maya-usd/blob/dev/lib/mayaUsd/fileio/shading/shadingModePxrRis.cpp
Actually ends up using a static mapping of keys and values.
https://github.com/Autodesk/maya-usd/blob/dev/lib/mayaUsd/fileio/shading/rfmShaderMap.h

This bring me to two points:

1. could the approach for this be simpler.
   This table makes sense for converting maya types to usdShade equivalents but the render man nodes the shaderMap essentially isn't really doing anything, the key and value are identical. It feels like if the node isn't in the shaderMap it could just lazily create the node in usdShade as-is or just check if the shader node is indeed a renderman node at all somehow.

2. I'm not sure how to go about making my own shadingMode in python, are there any examples for that knocking around or some guidance I can have here? It looks like it might be possible with mayaUsd.lib but its not totally clear how I would go about implementing that. Obviously I could look at compiling something custom but I'd like to avoid that.