It seems that sabnzbd uses my backup ($/block) server a lot more than if I was doing it manually. It theoretically only needs to access it if the par2 files cannot repair the bad parts.
An easy way to implement it might be to re-queue some rar files that failed the par2 check (but this time with a backup server enabled). It would be much more elegant if it could just go to server and get enough missing/corrupt pieces, but this would be significantly more difficult.
I can see the first approach being not too good if most of the files are slightly broken (it would need to re-download a lot of files). In my experience, though, the bad files are usually concentrated among just a few files.
This might be a long term thing, but I think it's worth considering. I'd always gladly trade a bit of extra processor time and free downloads in exchange for the downloads for which I'm paying per gigabyte.