There is several ways to do what you want to do. It will depend on your MATLAB knwoledge.
To answer you first question. You can access a property of any object (not only an app) if this property is a Public one (you can access Private property only in the method of that class).
I don't know your app so if several cause can set your proeprties to 0, I suggest you use events (advance MATLAB behavior but very powerfull).
If ther eis only one location in your app where this property is set to 0 then just call your script in your app after verifying the attribute is equal to 0