Is it possible to run a "reverse" cryptcheck?

Not exactly, but there are several pretty good workarounds to accomplish something similar:

  1. Hasher backend -- effectively allows crypt to support checksums. See these recent threads for some tips and tricks.

  2. rclone hashsum --download --output-file command, combined with check --checkfile. See this post.

  3. The latest beta of rclone bisync has a new --download-hash feature that may be of interest.

  4. The chunker backend can be used creatively as an alternative to hasher.

  5. I actually built the exact feature you're talking about one time... but kind of abandoned the experiment after seeing how slow it would be to use!

(6. I am also now working on something new which could be an interesting solution for this use case, when it's finished.)