Multiroom discretes go in through the multiroom port on the back panel. You will need a Xantech device to complete the hookup, a 794-50, feeding the Marantz from your remote rooms. The 794-50 (as I understand it) sends the same source codes as you would send to the front panel to the back panel port with the modulation stripped off. This would explain why you don't find separate discrete multi-out control codes.

The 794-50 works well, and is not very expensive. I have one on my desk right now for the next project.....