Not sure if you are using an Automated Workflow for Proof but we use a multi stage approach for our creative review process. Since we have multiple stages, we do have our first stage (which contains our proof reviewers) lock after the deadline passes. See screenshot for how this looks.
We have been doing this for about 4 years now with very minimal issues, as our reviewers are trained that their stage will lock if they miss the deadline.
Hope this helps!