Thank you jamie and wp (wp: click onto a listitem caption, not into the non-selection area).
After a lot of investigation i found out that in fact i was wrong (it was hard for me to believe that the click could not be fetched in the customlistview in difference to the regular listview):
In fact the click had been eaten up (differently to D7 however!) by a BeginDrag operation in the MouseDown event. Those lines were guilty:
if DragDetect(Handle, ClienttoScreen(Point(X, Y))) then
BeginDrag(True);
If i deactivate those lines, the click callback is back. So i might have a problem elsewhere, but the original topic is a false assumption (because it's due to interference with DragDop). So i need to apologize! I was not aware of that when doing the post.
Of course it might be the case that those special measures in Delphi 7 comctrls.pas i mentioned in my OP had been the reason why i did not encounter that in Delphi.