Hi,
if you can set up an input source area on your template screen then it should be no problem. I am not 100% sure if you can do that on a 400MXN but hopefully you can. You also need to feed a suitable signal into the screen from an external source such a a FreeSat box or similar.
Set the background time of the screen file to run for the overall time, say 1 hour (0-60 minutes).
Create an input source area the size you require.
Set the input source area source option to the correct input eg HDMI then set the run time to 5 minutes (0-5 minutes).
Create an image area in a new layer the size you want.
Set the image layer to run for 55 minutes 5-55 minutes).
Set the schedule to run for the time you want during the day eg 8:00am to 5:00pm.
Publish the screen file to the clients.
EricL