Hello Craig,
Although chaining MIDI devices via 'MID IN' isn't officially part of the MIDI standard, some devices do support doing so as an extension to the standard, including, as I understand, Classic MIDI Works devices. If it works reasonably well for you currently (when chaining via the CMKs), there's no reason that it shouldn't continue to work equally well (if still chaining via the CMKs) if simply changing the audio/MIDI interface, and/or computer, and/or Hauptwerk version.
Edit: P.S. If you do instead want to get a multi-port MIDI interface (which might help to avoid your existing ciphers), then the MOTU Microlite is the most popular amongst Hauptwerk users, which is well-buffered and well-proven. See also this topic, though:
viewtopic.php?f=3&t=18325&p=138108#p138108(Also, I don't know off-hand whether MOTU yet have a MIDI driver for macOS 11 'Big Sur'.)
Best regards, Martin.
Hauptwerk software designer/developer, Milan Digital Audio.