Another thought:

If you'd like to try to use the Sound Palette sliders, one experiment you could do would be to assign rhythm waveforms to a patch the way we've discussed before. Then the sliders will indeed address the tones of the patch, which are your rhythm instruments.

BTW: I did discover (with someone else's help) that you can't use the copy function to copy rhythm settings from a rhythm kit to a patch when you use rhythm waveforms this way. That is, say you wanted a certain cymbal from a certain kit to be assigned to tone 1 in a new patch. You can't use the copy function to copy that cymbal's rhythm-kit settings over into the patch/tone. Another bummer.

But good luck.