This is a fairly involved issue. Currently the video card gamma code only supports the main display (IE. CGMainDisplayID()) on OS/X. This needs to be enhanced to support multi-display OS/X machines.
I have posted on the Qt Centre forums describing the problem and asking about how this might be done. At this point I do not have a definitive answer but I do have some clues about how the problem might be solved.
The Qt Centre forum thread about this is located here
This thread has a fairly detailed explanation of what needs to be done and also has some follow up posts that give some clues about what needs to be looked at in the Quartz and Qt APIs. But these are only clues and what is there may be a dead end.