I’m working on a custom controller for myself for the MixPre ii. The way it is set up, it checks the USB PID and VID to see if it is one of the 3 controllers it supports, and then implements MIDI mappings based on that. If you plug a USB MIDI device that doesn’t have those PIDs it simply does not recognise that it has a control surface plugged in.
It wouldn’t be that hard to build a USB MIDI pass through device with a Teensy with the USB host shield, or simply using the MIDI in schematic on the PJRC MIDI library webpage. You could even change MIDI mappings to whatever you wanted, therefore using any controller you liked with the 8 series.
I’ll post photos of my project when it looks prettier.