Andy.

This should be fairly simple to achieve for you.
There may be some subtle differences depending on what version of Catalyst you are using but the principal below is the same.

Set your output to the LED Processor to be 1280x1024. You may wish to use an image pro or a DVI Parrot to confirm the EDID is accurate.
Set up your mix window on screen 2 with the resolution 280x200 and position it accordingly. (Flat Constant Aspect Ratio is OK)
(I suggest a test pattern with well defined lines so that you can see that the linup is accurate. You can trace panel outlines or similar. Use Adobe Illustrator or After Effects)
Load you clip into the layer and route it to your mix.
Select 'Unity Scale' from the FX Menu.

This should give you a pixel for pixel output. You can confirm this with your test pattern.

There are ways to do this with exact maps but it may be more complex than your needs require.

Hope that has helped.
S