There really needs to be one, preferably one that uses AES (because AES-NI) or similar, that will ship disk(s) with the data after a failure needed to rebuild the NAS. Maybe it just isn't profitable enough for their to be one?
One challenge is that most cloud backup services are competing in the "unlimited storage" market, and if I were running one of those, I would
not want FreeNAS owners as customers.
The most obvious (to me) cloud backup provider that isn't in the "unlimited storage" space is rsync.net (you can
zfs send directly to them), and their prices reflect the true cost of running and
supporting a high quality service.
A reasonable compromise is to backup one or more shares of the most important datasets from a client computer, using whatever service and software you prefer. I will once again plug Arq Backup in this context (just a satisfied customer), since it works with Amazon S3, Amazon Glacier, and Google Nearline, which are very inexpensive. I'm hoping it will also support BackBlaze's B2 when that comes out of beta.
Another option is to run either Amazon's or Google's command-line tools in a jail. There's an s3cmd plugin available, but I haven't tried it. Amazon's tools include a
sync command and Google has an
rsync command. Neither of these is as smart as true rsync, but it's better than manually figuring out the changes.