Hi Anthony,
I am going to try to shed some light on your plea from the wilderness for help. You are correct in saying the RHYTHM track is used in the STEP RECORD mode only. I have never used STEP RECORD. However, that doesn't disqualify me from offering some observations based on what my manual says.
The RHYTHM TRACK and CONTROL TRACK record the following events: Rhythm settings and selection changes; Fill-in 1 & 2; Intro & Ending 1 & 2; Start/Stop; and Tempo setting. As you are already aware, the CONTROL TRACK also records a whole host of other "events/selections". The TRACK ASSIGN function gives you latitude to change information on all tracks if I am interpreting what I read correctly. Therefore, it seems to me that RHYTHM information can be assigned to a track other than the dedicated (or preset) RHYTHM track, but if you do that, the information will be stored in the track assigned and not in the dedicated RHYTHM track. And if that is so, then you should be able to store non-rhythm information in the dedicated RHYTHM track using the TRACK ASSIGN function. My manual says "The CONTROL, APC, CHORD and RHYTHM parts cannot be assigned to more than one track." It does not say that the storage of that information is restricted to those tracks so named. I believe the situation may be different when using EASY RECORD or REALTIME RECORD, but I have no information about that one way or the other.
I am assuming after doing all of this reading that when you are using STEP RECORD, the CONTROL TRACK will not be storing rhythm settings and selections, fill-ins, intros, endings, start/stop events, or tempo settings because the RHYTHM track is storing that information. It wouldn't make sense (not to me anyway) to duplicate the storage of that information in the CONTROL TRACK as well.
Maybe Alec, Bill Norrie, or John North will want to comment and correct any misinformation or wrong assumptions I've given here. One thing for sure, though I have never used the step record feature, I've learned something by reading my manual and it may help me in the future.
I probably haven't given you the answers you are seeking, but at least it is a beginning.
Best Regards, Chuck