Is this package helpful at all https://our.umbraco.com/packages/website-utilities/lockout-membership-provider

If not, you could do something similar and roll your own membership provider.

Thanks, David

Looking for the User account locked for the Umbraco Back Office (instead of the membership)

Found it. If anyone needs to know, I've just hooked it to the OnApplicationStarting with the UserService.

public void OnApplicationStarting(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
    {           

        UserService.SavedUser += this.UserServiceSaved;

    }

And then

private void UserServiceSaved(IUserService sender, SaveEventArgs<IUser> e)
    {
        foreach (IUser user in e.SavedEntities)
        {
            //checking if the user is locked out
            if (user.IsLockedOut)
            {
                //do stuff here like email admin and user
            }
        }
    }

is working on a reply...