Nico,
Yes, there is a way to do this. In your callback where you install the systray icon, you can put in
SetSystemAttribute (ATTR_TASKBAR_BUTTON_VISIBLE, 0);
This will do the trick for you. Once you have maximized your app again you can use
SetSystemAttribute (ATTR_TASKBAR_BUTTON_VISIBLE, 1);
to make it apprear back in the taskbar when it's not minimized.