You may have noticed the Force Translate Shading Engine option in the export dialog (or in the Feature Overrides section of the Render Settings).

Force Translate Shading Engines forces MtoA to export shape nodes with a shader link, like this:

polymesh
{
 ...
 shader "aiStandard1SG"
 declare mtoa_shading_groups constant ARRAY NODE
 mtoa_shading_groups "aiStandard1SG"
}

where “aiStandard1SG” is the name of a MayaShadingEngine shader node.

This allows you to keep your shapes and shaders in separate ASS files. For example, you could have one standin that loads the shapes, and a second standin that loads the shaders. As long as the shape nodes include links to the shaders, Arnold will resolve the links and render the shapes with the right shaders.