When you say input not supported? Is this like a warning on the momitor when booted to windows PE saying that the resolution is not supported?
What kind of hardware is this on and what version of windows PE? I would test with the latest version of windows pe if what I am understanding you to say is that the monitor doesn't work correctly when in pe. Or alternativly an older version of windows pe if these are very old clients or monitors maybe the older version would provide better video performance for your hardware.
Maybe if you had a screen shot with your phone that you could post I would better understand the point of failure. Thanks