With a little more digging and remembering what I did last time, I'm able to answer my own question, for someone else who runs into this problem.

First, the guide to how to do this is in, "CCP Advanced Programming (MRX-1, IP, RS232 and Two Way)" pdf file.

What I was missing was that I also needed to put a feedback step in the macro list for the volume up/down and mute buttons.

Finally, the new DENONs continue to use port 23 for volume and mute.

Once those items were added/corrected, the Two-Way Volume Pop-Up on the MX-5000 worked as advertised.

I meant to post this earlier but the URC database has a lot of invalid entries for the AVR-2313CI (and other 13 models). It looks like someone just copied the 2012 model commands forward and some of the input select buttons have changed. There is also an "Option" button that will need to be learned.