I have been working on a related idea. Syncthing-fuse allows a node to view all files, but only stores a fixed amount of data locally.
In my mind, disk space is cheap, so finding one or more nodes that can hold the full index shouldn’t be a big deal. I wanted a client that could access all those files, but not necessarily store the data for all of them. So, I’m building the client! If you’re interested in helping, let me know!
Selective download and placeholder files discusses similar concepts quite extensively and includes my introduction of syncthing-fuse.
Syncthing-fuse will only support OSX and Linux. There has been some talk about Android supporting a similar feature in syncthing #193 Selective Sync and syncthing-android #81 Selective download.