In "advanced mode" where all 3 sections get to choose any instrument, add some special instrument options to the lists: aux-input, piano output, epiano output and sub output. So instead of the section being an instrument driven by the keyboard it becomes an audio patch and lets you chain the effects. This way you can add a lot more effects to a single instrument and you can use the effects for external instruments. Lots of fun live-patching and effects-modulation possibilities if you could do this one thing.
Probably have the aux-input pass-through disabled whenever it's being used as an instrument source. And same for section outputs, when they're used as a patch input, don't also mix them clean.
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Just FYI, in order to add effects to external audio sources requires major hardware changes... you need to convert A-to-D for starters... the input on the CP88/73 is not an A/D Input, it is simply an Aux In (which goes directly to the Main Outs).
if you are looking for a keyboard that can process external inputs in many innovative new ways, see the MONTAGE/MODX range of synthesizers.