If any of these would be possible, then it would be possible to have this virtual FS functionality pluggable to other existing software (and on systems without FUSE/root capabilities)

I'm implementing myself something similar in https://github.com/vadimkantorov/packfs - but I'm using libarchive under the hood which does not currently support seeks...

What would be eventually good is to be adding some seeking functionality to libarchive... (starting with ZIP and TAR archives)