Is there a way to have custom widgets as tooltips? Of course I could try to simulate the tooltip behavior, but it would be nice to have all look and feel aspects of tooltips, yet to make full customization of its contents possible.
Yes, there is a way.

Override the tooltip event(QEvent::ToolTip) on your main window or application.. and show your custom widget as tooltip.
You can also use a filter to catch the toopltip event . In such case you wont need to change existing code