Going back to the OP though, that sub-octave generator might actually be ok for what you want after all. I was playing around with it yesterday, and swapped my low-tuned guitar for my sons standard tuned ST-jr. It tracks pretty well when it's dropping a high note down on the one octave setting, though it gets messy with chords.

It just makes noise when I drop the three lower strings on my guitar down an octave, but I'm starting in drop C and I usually use the detune setting.

I only had a quick play last night but will try to record something on Monday when the kids are at school