A K-9 app-specific password would just create a false sense of security.
For example, one could just install an old version of K-9 that doesn’t have the protection feature. (there are a few more steps involved, but this is the basic idea)
While there are some exceptions, just take this as a rule of thumb: If someone has access to your unlocked phone, you have lost. App-specific passwords or “app locker” apps won’t help you.