Cyberjock had a great idea for this, which, to my surprise, has proven itself to be quite hands-off and elegant.
Get the app "Foldersync Lite" for your phones/tablets. Set it up on a schedule, wi-fi only, to certain SSIDs only (all standard functions in foldersync lite), and use the WebDAV service, of all things, for the service to FreeNAS. (That's how this came up---Cyberjock and I were talking, "Who the hell would need the stupid WebDAV service? Why do we even have that?" and we went out of our way to try something WebDAV, and accidentally he discovered this).
It really works amazingly well, to my surprise. Better than syncthing, various torrenting stuff, owncloud, etc.