Enable a second guest monitor and use Fullscreen mode.
With the guest shut down, in VBox Settings->Display, increase video memory and set the number of monitors to 2. Staart the guest. On the guest's VBox menu, check View -> Display 2 -> Enable.
Use the View menu and Display 1 and Display 2 to control which monitor each guest display appears on.
Also, I just discovered that Seamless mode only seems to be useful when I have more than one guest monitor. In that case, I can drag windows from the the guest onto the other monitor. With a Windows host, this does funny things to the desktop background and taskbar, but it works.