I noticed that although this is a popular concept, it has no real proper tag on danbooru. Underbust is a term used for garments of clothing that go around and under the bust, accentuating the breasts and revealing whatever might be worn underneath. This term can be used for bodices (post #2025381) corsets (post #2102684) or dresses (post #2092957) and can apply to many other things. No other tag can blanket this term as bodices normally cover the breasts and corsets have their own ambiguation problems, and this isn't covered at all by dresses under normal circumstances.
The biggest problem in applying this as a tag is obviously going back and tagging everything that's underbust as such, but a good start would be a manual look through of the bodice and corset tags. Efforts can also be raised in applying this tag to new posts and raising awareness of the tag in general, creating a wiki for it and so forth. Does this sound like a valid idea?