Killer idea. Brilliant in its simplicity. I'd like to add an 'exit custom' gesture to your scheme. That way, a sequence of less than the 16-step maximum could be entered. I don't think you'd need a 'rest' gesture (no input -> Learn). Substitute the 'Unison' reference.
My thought of MIDI Note input (with Note Off triggering the 'learn') seems overcomplicated by comparison. I think you'd still need a way to enter & determine what the 'reference base note' would be. All of the Harpeggiator steps are intervals, with mathematical relationships to the current pitch-detected 'base note'.
In other words, if an open high E was input as your first sequence step, you'd still need a reference note to determine whether it was intended to be a fifth, octave, etc. Two pitch shifters, so I'd hope for an A27: CUSTOM and B27: CUSTOM capability.
