IGES is a Vectorworks export option.
I'm not clear what bricad is.
IGES provides you with higher level geometry then the DXF-faces could. brlcad is an open source cad package, coming with converters (im- and export). You can convert IGES to brlcad to OBJ, which should produce very clean OBJ files. Than you can enjoy the great OBJ-support in Radiance, including smooth surfaces and all the neat stuff...
You can script the whole procedure.
CU Lars.