It would be nice to be able to set custom icons in QSystemTrayIcon::showMessage.

On X11 (and WinCE), a Qt widget is used (QBalloonTip), so custom icons can already be done in a custom widget style and reimplementing standardIconImplementation() for SP_MessageBoxInformation, and it would be easy to add a QIcon overload to showMessage to make this simpler.

But on Windows something else would be needed to be able to implement this, since a win32 API (Shell_NotifyIcon) is used for this internally.
Apparently it's possible to set a custom icon there too: http://msdn.microsoft.com/en-us/library/bb762159(VS.85).aspx talks about a lpdata member of type PNOTIFYICONDATA.