What I want to know is if it's possible for Windows to have accelerated graphics display on an output with no 3d acceleration. Eg. for a physical machine, you might have one of those USB GPUs with a second display on it, would it be possible to display part or all of a game window on that output, with the 3D work being done by a PCIE GPU and redirected back via the PCIE bus to the other output, even if it's a significant performance hit.
If that's possible, then you could have Xen pass through a discrete GPU, and also emulate the usual Cirrus graphics. The guest sees 2 heads, but you only really work on the Cirrus display with the discrete GPU doing 3d work and passing it back to the emulated display. Basically you could have a hardware-accelerated guest display in a window on the Dom0 desktop without having to dedicate a monitor (or monitor input) to it.
Like I said, it'd likely cause a big performance hit, but if it's less than 50% it'd probably be acceptable for a lot of cases.