I'm not sure if this is the same as your request, but Can BiaB do something like this already?

From the Help:
'The "Each Chorus that a song is playing, transpose by ___ semitones." setting will transpose the song up 5 semitones (a perfect 4th) after each chorus. You can use this in combination with the "endless loop middle choruses feature" to endlessly play the song as it transposes keys each chorus. This is a great way to practice a tune.'

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