Implement a ZFS cloud backup with minimal impact.
One way to do a ZFS cloud backup is to do full backups each time and since the s3ql filesystem has de-duplication it will automatically throw out the blocks it already has in the cloud.
There will be no hit on the WAN side but there is an extra cost in that it must scan all the blocks in the storage volume in order to know what needs to be backed up but it should work well enough for modest sized storage volumes and OK for larger storage volumes when backed up nightly or weekly.
The benefit is that the complexity is low and you can recover from any volume backup without having to cobble together incremental deltas. The problem with doing incremental backups is that it will make for a big mess at recovery time and coalescing/synthetic full backups are also a challenge.