I am marking this thread as solved, as now there is support for multiple shaders and proxy objects in BlenderBIM using IfcExternallyDefinedSurfaceStyle and IfcRelAssociatesDocuments. This allows artists to have workflows using standard shader languages like OSL and RSL, and high poly proxy objects, whilst still working with IFC as a native BIM format!
A demo of a linked shader is shown here:
An example of a proxy object is shown here where the high-poly monkey mesh is proxied by a low polygon cube which is stored in the IFC.
