Using snapraid you will only be able to restore files you have the parity off. So files added or modifications since a recent sync will be lost. Unless you have a backup copy...
As far as I know there is, currently, no tool as you describe it. There are backup tools with checksums that can be used to detect errors in backup sets, but not to fix them.
I am writing, as a "hobby project", a very simple command line backup tool, "bitback", for local snapshot style backups with file level deduplication and file hashes. It can also scan for, detect and automatically fix bit rot in both directions, to/from the most recent backup copy. No idea when it will be released. Perhaps this year...
The idea is to run bitback daily to backup and hash new or modified files in a timestamped full snapshot of the source filesystem. At the same time a certain percentage of the old files in both the source and the backup filesystems are "scrubbed" and checked for bitrot, and corrected if possible. For performance reasons (read speed during re-hash) the filesystems should be on different drives on the same computer, unless you have 10GbE or don't mind waiting and congestion on the network.
This sounds great! It sounds exactly like what I'm looking for.
Can I find you on e.g. github, so I can find it when/if it's released?
I think I'll go for something like scorch as a temporary solution.