luckily, you can implement the idea on top of existing systems. e.g. fuse or plan9/inferno's 9p/styx fs protocol.

"File indexing and searching for Plan 9", http://lsub.org/ls/export/tags.pdf describes mostly text indexing, but the interface to searching is through a file system (unfortunately not described in much detail at first glance).

only now do i remember that wrote something related once: attfs, http://www.ueber.net/code/r/attrfs. that's for inferno, using the styx/9p protocol. i never polished it, am not currently using it, and the interface the file system provides is not ready for casual file browsing, though that's mostly a detail (from an amount of code point of view). the things stored are strings though, not files. but the illusion that it stores files is easy to build.