I managed to find a solution to this.

By simply drawing the pixmap rather than the control element I am able to produce th desired results. The mouse press event for the parent widget can allow me to detect mouse clicks over the m_checkboxRect area and therefore maintain the checked state.

Qt Code: 
  1. switch (m_checkboxState)
  2. {
  3.     case Qt::Checked:
  4.         option.state = QStyle::State_On;				
  5. 	indicatorPixmap = QPixmap(":/Resources/indicator_on.png");
  6. 	break;
  7.     case  Qt::Unchecked:
  8. 	option.state = QStyle::State_Off;
  9. 	indicatorPixmap = QPixmap(":/Resources/indicator_off.png");
  10. 	break;
  11.     case Qt::PartiallyChecked:
  12. 	option.state = QStyle::State_NoChange;
  13. 	indicatorPixmap = QPixmap(":/Resources/indicator_partial.png");
  14. 	break;
  15.     default:
  16. 	break;
  17. }					
  18.  
  19. style()->drawItemPixmap(painter, m_checkboxRect, 0, indicatorPixmap );
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