I think this feature would be more than selective sync, adding the ability for the user to browse files, and (on demand) download them + exclude them from the ignorelist.
The owncloud app has something like that: you can browse all the files, you can open them, which syncs the respective file to the device, and you can also star them, which means future updates (on the server) will also be pulled.
Maybe also related: Webinterface to browse your files?