I guess it’s possible that someone has already built such a patch, and I’d love to see it. But I’m having a hard time understanding how to use the FFT objects in general, and I’d love some help building the first, analytical phase of this project.

Definitely check out all the I series Audio examples in the help system, that's mostly where I learned the practical FFT stuff from. Something that was hard for me to figure out about the real FFT is that it really only spits out the terms 1 to N/2+1, because the terms in the upper half are negative frequencies that are related to the positive frequency terms in the lower half. Term 1 is DC and term N/2+1 is Nyquist, and neither of those have negative frequency cousins in the upper half, so their coefficients represent the whole amount of those components, whereas the others only represent half (being split evenly between positive and neg frequency terms).

I hope I'm not making things more confusing.