But that doesn't lock them out of their account.
What scenario you have in mind?
Just to clarify, Master Password is just a concept of a secret that keys are derived from.
From what I understand you are worried that someone is using owner key instead of active without even being aware of. In such scenario it will stop working because owner key is no longer allowed to be used to do the active key's job, but in such case user still can use the owner key to set a new active key.

If however, a user have lost his owner key, there's nothing that could help, and nothing that upcoming version changes in the situation.