So I finally got around to updating Radiance for Mountain Lion, which was more difficult than it sounds.
The old version used the undocumented O3Manager API to control display brightness, but Apple removed it in 10.8. External displays can’t be controlled via the IODisplay API; instead, you need USB HID. The current implementation matches a display to a HID device by serial number; we’ll see how that works out.
If you’re interested in doing something similar in your own application, you can grab the new MDDisplayManager object from this page.