Here's the final working solution... Line 3 is the key... our parent class has a handle to a QTableView (m_pTrackTableView)
Qt Code: 
  1. bool SortFilterProxyModel::lessThan( const QModelIndex &left, const QModelIndex &right ) const
  2. {
  3.   const Qt::SortOrder sortOrder = ((MixxxView *)this->parent())->m_pTrackTableView->horizontalHeader()->sortIndicatorOrder();
  4.  
  5.   // Do the strings look like they are BPM counts?
  6.   const QString bpmPattern = "^[0-9]+\\.[0-9]+$";
  7.   if (sourceModel()->data( left ).toString().trimmed().indexOf(QRegExp(bpmPattern)) == 0
  8.     && sourceModel()->data( right ).toString().trimmed().indexOf(QRegExp(bpmPattern)) == 0) {
  9.     double leftBPM = sourceModel()->data( left ).toString().trimmed().toDouble();
  10.     double rightBPM = sourceModel()->data( right ).toString().trimmed().toDouble();
  11.  
  12.     if (leftBPM == 0.0 && sortOrder == Qt::AscendingOrder) rightBPM = -1 * rightBPM;
  13.     if (rightBPM == 0.0 && sortOrder == Qt::AscendingOrder) leftBPM = -1 * leftBPM;
  14. //    qDebug() << "BPM Comparasion leftBPM =" << leftBPM << "rightBPM =" << rightBPM;
  15.     return leftBPM < rightBPM;
  16.   }
  17. ....
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