This video clip mentions both "document-scoped bookmarks" and "related items". They seem similar but as I understand it they're not the same feature/concept.

Except for the TextEdit and RTF format they also mention an other example with a movie file and a separate related subtitle.

The API they mention is NSFilePresenter and the method "primaryPresentedItemURL".

https://developer.apple.com/library/mac/#documentation/Foundation/Reference/NSFilePresenter_protocol/Reference/Reference.html#//apple_ref/occ/intfp/NSFilePresenter/primaryPresentedItemURL

I don't know if these API can help in this case, but I think it's worth looking into. The behavior of The Unarchiver might be more dynamic then this API allows.

If this won't work here's another idea that will make it a bit more user friendly. When extracting files The Unarchiver will remember the last location where a file was extracted to. But if the last file I extracted was a RAR file, as explained above, The Unarchiver will instead remember the last path where I pressed "Search", not where it was extracted to. Can these to location be remembered separately?