Essentially you can't. Backblaze doesn't allow you to backup NAS[1], and as the devices in question can't run the backblaze software, you can't pay for them as separate computers.
Thanks for this answer! (Indeed it would have been too good to be true).
BTW I just installed Tarsnap on the ReadyNAS NV+, it works! It's really slow because the CPU on the NV+ is quite lame, but for simple text files it's fantastic and creates peace of mind.
[1]: http://www.backblaze.com/help.html#nas