You will have to fiddle around how to best make displaymanagers use it, but:
You can start your Xorg normally which will use your entire screen at full resolution, e.g.
Xorg :123 -ac
The -ac switch disables some (all?) of Xorgs restrictions it may have on allowing clients. Then you start Xephyr as the only client (you may want to adapt the resolution passed as -screen argument to your needs):
DISPLAY=:123 Xephyr :0 -screen 1720x1200
Xephyr is a sort of "overlay Xserver" which interacts with clients just like any Xserver, but instead of taking care of rendering and input (the user faced part), Xephyr relies on another Xserver to do that.
How you combine these commands greatly depends on your use case. In general, applications like session managers give you the opportunity to provide a command how to invoke an Xserver.
What I did to try it out:
/root/.xinitrc with the content
Xephyr :0 -ac -once -query myxdmcphost -screen 1400x1200`
where 1400 is my screen width -200 pixels.
Then I run
xinit -- :123 -my -usual -xorg -options -for -my -computer and as a result I get a login screen and a session that actually uses my whole screen but a 200 pixels wide column on the right hand side.
To have the same effect for e.g. GDM you would have to provide the
xinit command a couple of times in the
gdm.conf as the method to invoke an Xserver. But I can't tell you from the top of my head how to do that.