You can get a bit of this with a restore with –dry-run so it doesn’t actually write the files. It surprises itself when it doesn’t find them for verification though. For that, you could perhaps stream it through the hashing (if it’s not already done). One other concern I have is that a huge file could take a huge amount of work to totally verify…
I think the current approach of verifying just the dblock, dindex, and dlist raw volumes is weak proof of restore. Something closer would be better, and using mostly actual restore code might beat making testing-only code both on realism and on required work. Something sooner might be better than something fancy much later…