Hmm, I've actually never seen a Mac app do that. It might not a be bad idea if I can figure out how to do it, and find some time to do it.
The 7-zip code is, unfortunately, quite horrible. When I started out, I was considering using it, because it can extract a lot of formats, but I found it very badly designed, full of Windows-isms, and completely undocumented and uncommented. Well, I am guilty of that last part myself, so I can't really blame the author for that, but it did mean the code was unusable.
Right now I am using the 7z-C code from 7-zip, which is a separate implementation in plain C instead of Windows C++, which is much easier to use, but not as capable.