You could try to do your own thing with automation, lock the layer,
then use the event following the formula/cell change if an attempt to
unlock the layer is made. Popup a dialog to accept your password, if
not valid then change the later back to locked.
Thats true, but it depends on how secure it needs to be. Is this just
to prevent casual changes. Pretty much anything can be broken if
enough effort is expended.
An illusion of security is possibly worse than no security at all. If
the level of security is very weak then the person relying on that
level of security must be told that the security is low.