Register 16 outputs (DUMMY0..DUMMY15), which you can add modes etc using standard methods. First time you assign some mode, output gets "connected". Most standard configuration can be done with
xrandr tool (output enable/disable, position, mode etc).
There is also support for setting physical dimensions with WIDTH_MM and HEIGHT_MM output properties (xrandr --output ... --set ...) - I think that's the only non-standard part, because I didn't found any standard API for doing that (which isn't surprising, because normally it's detected from the hardware).
Fixes #1 (closed)