Note that Crashplan will allow you to offsite backup to another computer without paying them a dime. If you have a friend you'd like to exchange backups with, you can seed a disk locally (v. fast) then take it and plug it into a remote machine. From that point on it will only have to transmit...
One option worth looking at is Crashplan. There is a qpkg for it in the App Centre which will allow you to backup files directly from your NAS either to their cloud storage or another machine (remote or local).
The only caveat is that to get your account up and running (only the paid one offers...