That is a VERY good sign. If the tang is still alive with no treatment after this long, then I'm going to revise my guess and go with marine ich. Go read, then you'll learn that this has not gone away, you need to learn the life cycle of this parasite. I don't know how you'll treat the tang without any kind of QT. Your whole tank is now infected (of course, it probably always has been).
A tip, you are not married to putting fish into an aquarium. ANY chemically inert container will work, even a trash tub.
Is there a reason why no mods or anyone else has stepped up to help this guy?
At this point I STRONGLY recommend you go to wetwebmedia and search here on marine ich (WWM is much more easily searchable, in my opinion) for articles, etc. on treatment. If the fish were already in QT, then I'd say go with hyposalinity treatments first, then if the bug is strong, graduated carefully (especially in regards to Cu treatments with tangs) to a chemical solution.
If there is just no way you can QT the fish (at this point you're also looking at dismantling the whole tank OR putting the whole tank through hypo), then hit it with the BEST nutrition you can. If you don't have predators, maybe try a couple of Lysmata cleaner shrimp, although I've only really seen them go to work on larger fish than what most folks keep in their home aquaria, and your hands, they do a great job of trimming cuticles and dead skin.