I play Rolands and I've messed around with the Dynamic Arranger. I originally taught myself on organs and have plenty of bad habits! So I probably just press the keys in a lazy way, instead of "striking" them wrists-up the way you would have to on a piano. Bottom line, I don't have the kind of velocity control one needs to make Dynamic Arranger work effectively.
On the BK9 I sometimes use sliders to control Bass, Pad, etc., as the song builds. I've even messed with doubling the bass to a different octave on another style track. I believe this technique could be executed successfully "hands free" with a custom 3-part expression pedal. If you really want or need this functionality, that's the workaround I suggest you pursue.
Yeah, as you can customize the expression pedal's depth for each style Part individually per Performance, if you don't have the control to do dynamics following with your playing touch, you can always do it with your feet!
Plus you can have the pedal affect the ACC Parts much more than the rhythm section, so in effect backing off on the pedal can bring down the extra Parts almost completely, but just shave a hair off the rhythm section.
The only main difference is that you are bringing down the volume of the Part, but you aren't changing its dynamics (velocity), so there's no timbral difference. For me, it's this that makes the feature worthwhile. Real players change the sound as they play harder and softer, not just get louder or quieter.
Bad habits can change! In these times more than most, we have the time to work on things long shoved to the back-burner. A few weeks disciplined practice can do wonders if you focus on a specific skill and concentrate your effort towards it. Practicing the same lines played quietly, then played hard, rinse and repeat, soon enough you have sufficient touch to control the DA...