DaGurch,
You were on the right track.
You will just need to add the HDMI switching commands to the AFTER button on the particular changer's control page. This way for any particular movie it will call the before macro, then the slot and then the after macro for that changer, which will also set the HDMI input port for that particular changer.

Please let me know if you need anything else.

Best Regards,
-3B