If I were building a rig like this there are a couple routs I would look at. You could use GPU pass through with a capture device on the host OS. You can then display it on any of the host monitors in a window without the encoding overhead. A second option is to take all the monitors and put them through a matrix switch to switch between host and VM GFX outputs and route displays around. You still would need either a separate mouse and keyboard, or some mouse/keyboard sharing software to be able to navigate between VM and host.
The problem is many cheap HDMI matrix switches are very slow to achieve signal lock due to HDCP implementations. Proper broadcast grade gear like I am used to working with would rival the cost of most gaming computers, but could be done for a few $$$$. Those options still have frame rate limitations to 60fps though which could be an issue if you like high frame rate gaming.
I will be looking around for a GPU pass through instructional also, but there are a few things ahead of that on my todo list.
Edit: instead of using a capture device or matrix switch, take a look at Looking Glass. Not sure if they support multiple monitors yet, but it would be the way to go.