Despite your email from iZotope, I think it might be worth playing around with the Spectral Repair tool in RX. It's pretty amazing what it can do, and while I don't think it will get you what you are looking for exactly, it could possibly allow you to select part of the vocal, extract that as a new item and process it and then play that separate item alongside the original. Kind of like the idea behind parallel compression. This is likely to be highly tedious for anything but a short excerpt though.