I use seafile for my roms and foldersync for saves and other stuff. Best of both worlds, the seafile client on android works like… google play music, you select an item and it’s cached locally. I think dropbox on android works the same.
So I have my playlists, settings, saves and states synced on my desktop, my shields and phone, and I just pre-load the roms I want to play.
Btw there is a trick you could use if you have a server to have instant access to your roms that are not stored locally. You could make a web server with your roms, and generate a raw list of the dir and name it .index, like this: http://builbot.libretro.com/assets/cores/gw/.index
Then change your content downloader URL to your server URL and you would have instant access to your roms on your server from within the application. The only thing we need for that to be perfect is to be able to navigate subdirectories, there are already provisions for that but it’s not finished