I know this is a complete diversion (in a way) from the backup software you originally mentioned, but well worth considering:
I don't know if you have to stick with the "dumb" nas box you mentioned....
If not, take a look at one you can run *nix on, like the qnap or synology nas boxes.
They tend to be expensive, but are very good.
Or even one of those little NSLU2 devices - they can be extremely versatile and are a much cheaper way to go, and use cheap USB drives.
I have one running a full Debian install, via an attached 2.5" caddy.
They are more powerful then they might first appear. I even got it running x sessions over ssh at a usable rate, just for fun, which is surprising for the spec of the little thing!
Just some food for thought