I’ve just tried again to double-check, and I think I found out what’s going on here!
It’s actually possible to use all 9 Parameter knobs to send MIDI, as long as they’re controlling continuous values on the associated engine on the Synth (more on this later). Let me explain with an example.
The simplest way is by going to Scene and selecting Init; this will create an empty scene with an ACD synth. If you go to the Engine section, you’ll notice all 9 Parameter knobs control something here; now open Ableton (or any other DAW) and assign each knob to control something via MIDI. You’ll notice they all work, apart from the rightmost knob in the middle row: if you look at the Synth screen, this knob controls Sub Mode, which has discrete values (-1 Oct Square, -1 Oct Pulse, etc); for some reason, when knobs control discrete parameters (or when they are unassigned) you can’t use them for MIDI. In fact, if you open the MIDI settings section on the Synth and choose Control Change mapping, you can see the CC number associated to each Parameter knob.
I haven’t found any synth engine where all 9 knobs are usable (either some are unassigned or some control discrete parameters), so I think it would be useful to have a “MIDI synth engine” where all 9 Parameter knobs can be used to send MIDI signals, it would be a cool feature!