IGES has been designed to import/export CSG models, then Brep has been added.
OpenCASCADE read CSG IGES (build the CSG tree) but immediatly compute the Brep that you import.
1/ Maybe you can use this CSG tree to import directly CSG models from IGES.
BRLCAD do the same thing http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA274013 (not with OCC !).
2/ Maybe you can also export CSG IGES files to CAD systems.
3/ There exist also an XML schema to describe such geometries: GDM ( http://gdml.web.cern.ch ) used by High Energy Physics Communities (CERN, FERMI lab, ...)
Thanks for you attention
1/ and 2/ should be inserted in OCC IGES reader even if there not usable by OCC today.