I think pseudo-folders or virtual folders a la spotlight should not be difficult to implement. You could see this as a simple shortcut to a search query.
It would be really nice if this was possible.
If one could "plug" these things into the filesystem, it would be "easy" to keep up with new "archive" type file formats.
This way the user can browse through these archives like it browses any other folder and, as an added bonus, the IndexFeeder could also index files (usually "hidden" inside these archives) seamlessly without any need for special IndexFeeder plugins to understand some archive files.
If the archive file format supports easy implementation of on-the-fly write-in, the better (like ZipFolders did on Windows, and I think XP does also with .zip files only... XP calls it Compressed Folders). If not, at least it would present the user with a read-only "folder".
To me, *all* OSs should deal equally with directories and archive type files because all these are (compressed or not) *folders*, from a user point of view.
So, instead of using a SkyZip app, a SkyRAR app, SkyUNTAR app, etc... or mount/unmount a .iso file, all these files could be dealt by the filesystem plugin (or at least by the filesystem browser app plug-in).
I see the use of a specific app for some archive formats only when, for a performance or implementation reason, the plug-in philosophy couldn't deal with the update of that archive file.