How should I insulate a cantilevered bay with plumbing, ducting, and electrical?
I'd first be concerned with freezing pipes. If that's not an issue in your climate I'd pay to have the whole thing spray foamed. That's pricy but a good R value and prevents air leakage. Otherwise I'd use rigid foam and try to fill all the spaces and voids, with Great Stuff to "glue" it in place. That would be cheaper than spray foam but a lot more work as well.