There appears to be no simple function for setting pixels values. I can see loads of different ways of doing this but for symmetry sake what about something like this alongside rectangles, lines and circles?
void cvPixel(CvArr*, CvPoint, CvScalar);
void cvPixel(CvArr* img, CvPoint pt, CvScalar color){ cvSet2D(img,pt.y,pt.x,color); };