If anyone else is playing with this, here is the post I saw on the mental ray beta. Its a little out of date but gives you an idea of how to build an iray scene.
Here is a little tutorial how to adjust your scene inside Maya and tune export options, so that the final .mi file can be used successfully with Standalone in iray rendering mode.
Maya manipulate scene for iray
Introduction
A Maya scene can be tuned to use only a small subset of shaders and effects which is supported by iray. Different versions of iray handle certain shaders differently. For example, the Maya shadingNode (also known as Shading Engine) may be properly detected and skipped by iray, or, iray shader mapping will crash.
Prepare Scene for iray
The iray shader mapping supports a list of shaders, see release notes for recent status.
Currently suggested shaders are:
mia_material, mia_material_x as material surface shader, disable Multiple Outputs
Maya file node, or mental ray base shaders, for texture image lookup
mia_exposure_photographic as camera lens shader
mia_physical_sky as camera environment shader
physical_lens_dof as camera lens shader
Export .mi File for iray
In order to create a .mi file for rendering with mental ray Standalone, please follow the following guidance to export .mi file without Maya specifics.
Maya export .mi files without Maya specifics
Contents
1 Introduction
2 To create non-Maya specific scenes
3 Tune global translation options
4 Tune file export options
5 Tweak exported .mi file
Introduction
Sometimes it is desired to create scenes which just contain mental ray shaders, like those from architectural, base, paint, physics, and subsurface libraries, and no shaders from Adsk or mayabase packages. Using just those shaders is possible, but upon export to an .mi file there is lots of other custom Maya stuff included. To get rid of this the following steps need to be performed (manually or per script).
To create non-Maya specific scenes
Do not use native Maya material shaders
Assign materials from Lighting/Shading / Assign New Material menu in the bottom section only
After assignment, the Attribute Editor (AE) opens for the shadingGroup (SG) node
In Custom Shaders section
Suppress all Maya Shaders on
Export with Shading Engine off
Export with Volume Sampler off
Do not enable certain Maya visual effects
Depth Of Field (Camera / Depth of Field), instead attach mental ray lens shader in the camera
Image-Based Lighting (Render Settings / Indirect Lighting / Image-Based Lighting), instead attach environment shader in the camera, or use Physical Sun and Sky
Tune global translation options
In Render Settings (Window / Rendering Editors / Render Settings)
Common / Render Options section
Enable Default Light off
Options / Translation section
Export Particles off
Export Particle Instances off
Export Fluids off
Export Post Effects off
Options / Customization section
Use Legacy Maya Base Shader on (Maya2009 and up)
Export State Shader off
Export Maya Options off
Export Custom Colors off
Export Custom Data off
Export Custom Vectors off
Tune file export options
In Export Options (File / Export All… / Options…)
General Options section
File type: mentalRay
File Type Specific Options section
Export selection output: Renderable scene
Export file paths: Output image: none
Export selected items only: off (see below)
Tweak exported .mi file
Maya2009 and later always include Adsk shader references into exported .mi files. mental ray will emit an error message which can be ignored, it does not stop rendering.
But there are ways to resolve the problem from within Maya.
Enable filter in Export Options for include and link statements, to not write them into .mi files. Load shaders in the external application instead.
Provide custom shader libraries in Render Settings / Options / Custom Globals to override default translation, which affects both integrated rendering and export.
Use Render Settings / Options / Custom Scene Text to provide custom libraries to only affect .mi file generation but not rendering.
