I’m glad you found a solution within Gig Performer that fits even better than your original idea.
Still there might be some use cases for it, like when your gig file has been set up for 88 keys but you temporarily only have a shorter keyboard on vacation, in the office, on the toilet… - so this would be the answer to your initial question:
In this case you would use only one MIDI input whose transpose is controlled by a widget. You would then do the splitting with “fake” MIDI In blocks (like the MIDI In OSC) that are only used for key zones and filtering.