I wrote an ArchiCAD to Radiance exporter some years ago. That was using ArchiCAD 8.0 and Visual Studio 6.0 in Windows 2000.
After some (serious) struggling to get my develoment environment up to date and working again, I finally succeeded compiling my add-on again in an updated environment: using ArchiCAD 16 on OSX 10.7, XCode 3.2.6 (dug up from underneath a pile of Time Machine dust) and an up-to-date Radiance distribution (with now a native rvu viewer to replace rView that works natively in OSX using Qt instead of X11).
There is lot of things to polish, still, but the geometry and color information seems to survive OK. I had to revise the transformation of the geometry again (see discussion in
http://archicad-talk.graphisoft.com/viewtopic.php?t=3938).
It is not ready for release and still have to test in Windows too (only have a 32-bit environment available at the moment, bursting at the edge of filling my boot camp partition, alas).
But want to thank this forum for some help along the way and want to suggest GS to do some more efforts to make the API more accessible (update compilers, more examples, double-check examples in a clean install, ...). But definitely more open examples in the Documentation as well. There is so little info available, e.g. when comparing with ADSK.